The blue tie is thrown carelessly over the right shoulder to prevent his lunch of Marks & Spencer pasta salad and Diet Coke – "Do you mind if I munch?" – being spilt on it in the back of the yellow bus as it swings along the M40 towards Oxford. It enhances his boyish appearance, and a disconcerting sense that, at 43, Nick Clegg looks even younger than when he was energetically pushing his ideas as a little-known MEP a decade ago. Between mouthfuls, Clegg is patiently explaining to me that the Liberal Democrats' extraordinary opinion poll surge, by now extended for two weeks, is not some "inexplicable flash in the pan" and that "something was brewing" for some time.
"In the time you've been away, who would have thought that the smaller of the three main parties, the Lib Dems, should now be geographically the most evenly spread?" In the last two elections, "more people didn't vote than voted for the winning party, and the dam was bound to burst at some point". While accurate, this analysis understates his own role. But Clegg can afford to be modest. By now, of course, he is instantly recognisable; when he arrives at Oxford Brookes University the hundreds of students packing the hall greet him, if not like a rock idol, with an excitement seldom, if ever, generated by a British politician in these times. The journalists squeezed into the front of the bus include two senior German correspondents urged by their offices to explain the Clegg phenomenon.
When our conversation is over, he strolls down the bus and is engaged by them in a reflective discussion on the trouble Britain has had adjusting to its relationship with the European Union. He suggests that the problem lies with Britain's belated entry; by not being among the original six Common Market members she had not shared in the triumphant sense that its formation had been a decisive assertion of European peace over war; instead she saw it more as an admission of her own failure any longer to dominate the world stage.
The exchanges show how comfortable Clegg is in his intellectual skin. Most of his predecessors saw the EU as he does, while acknowledging all its faults, as he also does. But they might have been more wary about expressing their Europeanism so freely in the middle of an election lest it upset the Eurosceptic supporters the party has in regions like the South-west of England. Clegg is equally relaxed when he gently mocks the Daily Mail's obsession with his in-laws. "Both my wife's parents are Spanish—shock horror. She is er... Spanish." Or when at other times of the campaign he confesses not to believing in God, or when he sticks boldly to his advocacy of an amnesty for illegal immigrants under double pressure from Gordon Brown and David Cameron. It is as if Clegg realises, at least subconsciously, that his potency as a national electoral force now outweighs the advantages of a politician's caution. And sure enough, when he leaves the Oxford hall after his question-and-answer session, the huge amoeba-like mob of TV crews surrounding the Liberal Democrat leader on the lawn outside testifies to the fact that Clegg is the undisputed story of the 2010 election campaign.
But what we are seeing here is much more than one man's stardom. Is it possible that Britain's third party is about to realise the goal that eluded the Social Democrat-Liberal alliance nearly a quarter of a century ago, and beat Labour into third place in vote share? And that it will do so without being afforded the space offered its predecessors then by the combination of a Tory Prime Minister, Margaret Thatcher, who was hated by as many electors as admired her, on the one hand, and a deeply unpopular Labour Party, unilateralist, infected by the "loony" left, rebarbatively tax-raising, on the other? And even that the shift in the tectonic plates of politics is so seismic that history is about to come full circle and, having first spawned and then been overtaken by the Labour Party in the 20th century, the Liberal tradition in British politics is about to become once again a hegemonic force in the 21st?
It all seemed so different before the "dam burst". A mere 16 days – and another political era – earlier Clegg had arrived with the minimum of fuss – and only a handful of reporters – at the Roundabout Club, a successful play and after-school service for three to 15-year-olds in Northampton, started and run by two local parents – and Liberal Democrat councillors – Vanessa and Roger Pine. The Liberal Democrats were trailing in the polls at a familiar level of 19.5 per cent. It was only three weeks since the ITV interviewer Mary Nightingale had teased Clegg on air that he had walked through Waterloo Station without a flicker of recognition from anyone.
Clegg was here chiefly to support the local party in the target seat of Northampton North, where the local candidate, Andrew Simpson, has come from third place to within sight of winning. His visit was welcome, of course, but his party's popularity in the community has much less to do with him than with the excellent service provided by the Pines. As a result of it, Karen Dhlamini, 35, has started a psychology degree. "I couldn't have done that without them, and yes, I'll be voting Liberal Democrat."
Clegg chats easily to the mums and their impatient children – "Has it finished?" asked a plaintive five-year-old Aaliyah Mossin to laughs – before leaving for local media interviews. But so-called Cleggmania there isn't. In fact you understand why the Liberal Democrat bus is decorated with pictures both of Clegg and his shadow Chancellor, Vince Cable. At this stage, back in mid-April, Cable was the most popular politician in Britain, his party's one indisputable electoral asset, one with which a leader about whom the public was still hesitant badly needed to be associated.
What changed all this, of course, was the seminal moment of the campaign: the first television debate. This did not happen by accident. Clegg has led the charge against conventional politics. Of course, he is a politician, and election campaigns can hardly fail to raise the profile of a third-party leader as it had the last time a hung parliament loomed, propelling an under-recognised Jeremy Thorpe to sudden national prominence in February 1974. But Clegg had struggled more than most: to make an impact at Prime Minister's Questions, to force his way into the national debate from the Today programme to Newsnight as Paddy Ashdown and David Owen had been such past masters at doing. So as soon as it looked likely, back in January, that the debates would happen, he saw it, as one of his advisers would later explain, as a "massive opportunity".
He began work immediately: he was helped by the dozens of town hall meetings he had held since becoming leader, which had given him the sense that the polls might be understating the underlying discontent with the two main parties and the potential appeal of a political establishment "outsider", particularly after the MPs' expenses fiasco erupted in late 2008. He was helped too by his own picks as strategists of John Sharkey, a top advertising man who had worked for Saatchi & Saatchi on Mrs Thatcher's campaigns, and Chris Fox, a former corporate communications executive with wide experience of industry.
US TV debates were studied exhaustively; while everyone remembers Lloyd Bentsen telling Dan Quayle "Senator, you're no Jack Kennedy", the Quayle debate that senior Lib Dems studied even more closely was one four years later with Bill Clinton's running-mate Al Gore, in which Gore repeatedly maintained the attack by slowing down changes of subject by the moderator: "I'll answer that, but first let me deal with the point made by Vice-President Quayle..." By the time Gordon Brown had called the election on 6 April, there had been several full-length dress rehearsals, featuring every permutation of attack and counter-attack between the three leaders – the videos of which Clegg himself took home for further analysis. The mantra all took to heart is one coined by the late Lord (Richard) Holme about third-party politics: "The best shot is the single bullet which gets both the other two."
Yet Clegg had prepared for this moment far longer than that, in a career accelerated by a combination of luck and judgement. He was talent-spotted by Paddy Ashdown, who first heard of him when Clegg was working for the Tory European Commissioner Leon Brittan. After he became an MEP in 2000, Ashdown told some of his closest friends in the party to nurture him as a potential future leader. One who did believes that his marriage to Miriam Gonzalez was crucial in helping to decide on seeking a Westminster seat. "He became more focused and confident," he says. It was luck that Rick Allen the Liberal Democrat member for Sheffield Hallam, decided shortly after the 2001 election that he didn't want to stand again. But it may have been judgement when Clegg decided not to run against Menzies Campbell when Charles Kennedy stood down in 2007; the danger was that of being stamped as a losing candidate.
Liberal Democrats who have worked closely with Clegg suggest that the Northern urban base is especially important to him (though his actual constituency is one of the most middle-class in the country) precisely because of his international – or, as the Daily Mail would put it with a touch of impeccably phrased xenophobia, "cosmopolitan" – connections. Both in the leaders' debates and on the road, many of Clegg's references – to social deprivation, say, or industrial change, or housing – are rooted in the city he represents. "Sheffield is his Sim City," says one colleague.
The massive structural deficit, most of it the product of the huge bailout to save the financial system, has rarely been centre stage in this election because of the tacit accord between the parties not to be specific about the dire tax rises and spending cuts that will be needed to cut at least £30bn of it. But it hangs over the campaign, not least because many electors were more aware of it than the politicians gave them credit for.
At Oxford Brookes it is striking that the audience member who asks Clegg about the Liberal Democrats' policy of phasing out university tuition fees, Tori Hill, 21, specifically mentions it. Clegg's answer is characteristically skilful, arguing that with record levels of personal debt it made no sense to create "more debt". But Hill is not 100 per cent convinced. "I think it's a great idea to end tuition fees but I don't know whether there's going to be sufficient funds to meet the demands of the economy," she says afterwards. When it's put to the students that they seem more conscious of the tough times ahead than the politicians, another, Robyn Stock, also 21, says succinctly: "They've got jobs. We haven't." But Stock supports the Lib Dems, responding to Clegg's specific appeal in the wake of the MPs' expenses row: "I was so angry about it," she says. "Paying for cleaning your swimming pool – how could anyone think that was OK?"
Certainly the Lib Dems' programme is not beyond criticism. The commitment on tuition fees, though phased, would force even deeper cuts in public spending. The party admits that its balance of tax increases and spending cuts to tackle the deficit is more weighted to the latter than the Government's. The costings of the gross £17bn tax cut – much criticised by the other parties – were last week given a relatively clean bill of health by the Institute of Fiscal Studies. While the IFS doubted that the party's targets for stamping out tax evasion would be met, it thought its planned increases in capital gains tax could raise more than envisaged. But though the measure has largely been presented as helping the low-paid it will actually help upper middle-income double earners the most. Nor is it clear exactly what the commitment to rule out a Trident replacement – possibly a hangover compromise from the leadership contest when Chris Huhne wanted Trident scrapped and Clegg wanted to keep it – really means. If a nuclear alternative is envisaged it could be as costly. If the Lib Dems are determined to go non-nuclear they are not saying so.
That said, the Lib Dems have been the boldest in their undoubtedly popular post-crash onslaught on the banks, including the plan – with an interim levy on them in the meantime – for a separation of investment and retail banking, one favoured by both Mervyn King, the Governor of the Bank of England, and President Obama. Moreover Cable has a better record than his counterparts in questioning the Government's lack of regulation of the financial system in its earlier years – including his attack on demutualisation and the recklessly credit-financed housing boom. Nor is it confined to the banks. In the bowels of the Lib Dems' Westminster headquarters, I asked Cable whether he regretted making front-page headlines in the first week of the campaign by describing as "nauseating" the Tory-fuelled attack by business leaders on the Government's planned national insurance increase. He hadn't been attacking all of them, he explained, let alone small businesses. "But ... there were some companies whose remuneration packages for their chief executives were larger than the cost of the national insurance increase. For these people to be spluttering with indignation was indeed nauseating." And, yes, he was very happy that Clegg as leader had now taken over his star mantle. "Rising tides lift all boats," he said.
Above all, it's Clegg's coherent attempt at permanent reform of British politics that pervades this campaign. The extent of his radicalism in the last few weeks has not yet been fully appreciated. With his repeated dismissal of "some dusty constitutional convention" that seats won are the only indicator of success, and his insistence that Brown will not be able to continue as Prime Minister in any deal with the Lib Dems if Labour has come third, Clegg has actually introduced an element of virtual proportionality into what may be the last "first past the post" election (one which incidentally may make some Labour voters chary of voting Lib Dem tactically in seats where Labour is third, since Labour's national vote share could be relevant to what kind of government takes office after Thursday). He is indicating only that the stipulation on Brown in third place, and the sensible assertion that if the Tories had both a majority in seats and votes they were entitled to form a government, were the only parameters he was laying down for a possible hung parliament. Within them, in other words, anything is possible, ranging from a coalition to allowing another party – almost certainly the Tories on present showing – to pass a Queen's Speech as a minority government.
Cable would also not discuss post-Thursday scenarios, beyond saying that the Lib Dems would push hard their plan for a cross-party financial stability council as a means of achieving some form of consensus on the scale of the deficit-cutting task ahead. He did, however, recommend an article in the Evening Standard by Lord Owen midway through the campaign. It's impossible to say what Cable liked about the article because he wasn't saying. But it certainly makes interesting reading – including the suggestion that the Lib Dems might insist on Ken Clarke as Chancellor in coalition talks with the Tories, with Cable becoming Business Secretary.
Owen suggested that while Clegg was right to start talking to the party with the "largest mandate", he was fully entitled to turn to a second one if he failed to secure a pact with the first "that reflects the electorate's vote". And if that was a Labour Party which had "lost many MPs and failed to attract the most votes", then he would indeed also be entitled to insist on Brown's removal. It is not clear what, if any, precedent there is for this, but Owen argued that Clegg's distant predecessor Thorpe made a "great mistake" by not similarly calling for Edward Heath to go when he was considering a possible coalition in February 1974. The problem, of course, with that is the lack – so far – of an agreed Labour successor, which is why several ministers believe that if Labour comes third in votes a deal with the Lib Dems is a non-starter. The only counter-possibility canvassed in some Labour circles is that Brown would offer the Lib Dems a more proportional "AV plus" electoral system, a sizeable number – well over half a dozen – of key cabinet posts, and a future timetable for his departure.
Clegg has given no indication that this would be acceptable. Indeed, he told me in the back of the bus to Oxford last Wednesday that "after 13 years you run out of steam, you get hollowed out, you lose your way and I think the Labour Party needs to recharge its batteries by going into opposition". He accepts what he calls the "first part" of the analysis by the late Lord Jenkins that Labour and the Lib Dems together form the "progressive wing of British politics... in the broadest sense of the word".
What he doesn't accept is that this means the Lib Dems should become an "adjunct" of Labour. Arguing that his party is ahead of Labour "not of course in terms of bums on seats but in the battle of ideas" on the "progressive tests" of "liberty versus authority, individual versus the state, nationalism versus internationalism, individual mobility versus dependence on the state, political transparency versus the hogging of power", he adds: "I've always rejected the complacency and the assumption by many people in the Labour Party that they have the first claim on the progressive instincts of fairness of the people in Britain. I really don't think they do."
Clegg knows that "what goes up can come down" – and indeed two of yesterday's polls show his party in third place. But the first achievement of Clegg's campaign may have been to go further than ever before in persuading voters that his party can genuinely make a difference – that a Lib Dem vote is no longer a wasted one. We will soon know whether that perception was right. The second achievement may well be finally to make electoral reform the mainstream cause it failed to be for a century or more.
Tomorrow: Donald Macintyre on David Cameron