John Rentoul: Be careful what you wish for, Mr Clegg

The Lib Dems could end up holding the balance of power in parliament just when it suits them least

The Liberal Democrats are a surprisingly resilient force. When David Cameron became leader of the Conservatives he took away two of their three distinctive policies: the environment and civil liberties. The third, Iraq, was already losing its potency by the time of the last election.

Without any obvious reason to vote for them and at various stages without a leader – because Charles Kennedy was unable to leave his Commons office because of a "stomach bug" or because Vince Cable was standing in – the party continued to win local council elections and to fare reasonably well in opinion polls.

Then the world was turned upside down – in the middle of last year's Lib Dem annual conference, as it happened – when Lehman Brothers went bust. Suddenly, the Tories became the party that opposed tax cuts. And New Labour, a project founded on the abolition of the old Clause IV, found itself enacting the old form of words, as it took common ownership of, if not the means of production and distribution, at least those of exchange. As John Prescott said to Alastair Campbell, as the two walked past the banks at Canary Wharf after a Labour fundraising event last weekend: "To think, we always dreamed that one day this would be ours ... ".

As for the Lib Dems, having lost greenery, civil liberties and Iraq, they gained the economy. Vince Cable became famous as the one MP who had warned of the credit bubble – almost as famous as he had been for his ballroom dancing. He was ahead of the pack on Northern Rock and has a kind of non-political credibility on the wider crisis. And what effect did this priceless political gain have on the third party's standing? Not much.

The party has been mostly at between 15 and 20 per cent in the opinion polls since the last election. The two big parties have gone up and down as leaders, prime ministers and economic crises come and go, but the Lib Dems trundle on.

They are trundling towards their best chance of a place in government since the Lib-Lab pact between James Callaghan and David Steel dissolved a generation ago. When Nick Clegg speaks to Lib Dems in Harrogate today, he could tell them, without sounding ridiculous, to go back to their constituencies and prepare for government. At the moment it might look as if David Cameron is heading for outright victory at the next election, but funny things happen in politics. And the peculiarities of the British voting system, and voter behaviour, mean that the Conservatives need to be eight or 10 percentage points ahead to scrape a majority in the House of Commons, which does not give Cameron much margin for error.

Much concentrating of minds going on, therefore, in both the Tory shadow cabinet and the Lib Dem equivalent. David Cameron and his "preparing for government" supremo, Francis Maude, have met Gus O'Donnell, the Cabinet Secretary, to open discussions about the mechanics of a possible change of government. So has Nick Clegg.

The chance to talk to civil servants before an election was turned down by Kennedy when he was leader, but Clegg "jumped at it", according to one of his advisers. It makes sense, because it reinforces his message that he is "hungry for power".

The Lib Dem leader has positioned himself astutely for the possibility of a hung parliament. He has done two important things. One, he has reinterpreted the old Paddy Ashdown policy of equidistance. He told me last week that, if neither Labour nor the Conservatives won a clear mandate, the Lib Dems would not seek to decide between them. "I don't think we're going to be playing eeny-meeny-miny-moe in British politics," he said. He would look first to work with whichever party won more seats.

Clegg may not sound very equidistant in his speech today. I am told that he will lay into Labour – for its failure to deliver more by way of social justice after 12 years; for Brown's desperation to be seen with President Barack Obama rather than to work with the European Union; and for the gap between the green rhetoric of ministers and the grey reality.

That is because Labour is in government, although it may also be to offset the hostility of much of his party's grassroots towards the Tories.

The second thing that Clegg has done to prepare for a hung parliament is to say that electoral reform is "not a deal breaker". This is sensible, because it gives him greater flexibility. It also avoids giving the impression that his party is obsessed with changing the rules of the game in its favour, rather than serving the people. But it remains a paradox that, at a time when the electoral system seems more distorted than ever, and when people are more disillusioned than ever, there is so little demand for a more proportional system.

What, though, would negotiations in a hung parliament be about? Not, I predict, about this weekend's policy document on further and higher education that commits the party, as a "key principle", to "the creation of a climbing frame for learning". Judging by Clegg's priorities this weekend, the Lib Dems are going back to their basics: greenery, on the basis that the Tories are losing interest as the recession bites; Europe; and social justice, now renamed social mobility.

The difficult issues between the parties after the next election, however, are likely to be those staples of British political contention: tax and spending. Here it gets cloudy. Just as the economic crisis broke at the Lib Dem conference last September, Clegg sought to define the party as that of tax cuts for many and tax rises for the few. This weekend his tax message is that he wants to cancel the VAT cut immediately. After the next election, though, the issue will surely be how honest any of the parties are about the big tax rises that will be needed from everyone to get the public finances back on track.

On that issue, it could be the worst time in British politics to have a hung parliament. The worst time for the Liberal Democrat dream of a share of power finally to come true.