John Rentoul: The anti-politics party is ahead

A by-election flushes out mid-term anger, but Her Majesty's Opposition is annoying voters too

I don't know if some of my colleagues in the journalistic trade are having a laugh, but The Spectator wrote last week that, if the Liberal Democrats won the Oldham East and Saddleworth by-election, "it would transform politics". A surprisingly early contender for understatement of the year. It would not just transform politics, it would defy the laws of human nature, physics and the universe.

One enterprising bookmaker, Paddy Power, last week put out a news release that attracted some publicity, saying that the chances of Elwyn Watkins, the Lib Dem candidate, winning the by-election were the same as those of Manchester United being relegated. Some killjoy pedant pointed out that this comparison was not supported by the odds that Paddy Power itself quoted, which gave Watkins a 30 per cent chance of winning. But the news release was more accurate than the quoted odds. The idea that the Lib Dems have a 30 per cent chance of victory is, of course, balderdash – a technical term for a bookies' profit margin. It is strictly accurate, therefore, to say that a Lib Dem win in Oldham would be helpful to Nick Clegg. It would be helpful in the way that Harry Potter benefits from the discovery that he is the most famous wizard in the world. Unfortunately for Clegg, being locked in the understairs cupboard of public opinion is no guarantee that a hairy giant is about to burst in and take him off to Hogwarts.

Indeed, the voters of Oldham are more likely to adopt the role of Uncle Vernon and subject the unfortunate prisoner to further humiliation. Today's Sunday Telegraph poll in the constituency suggests that the Lib Dems are some distance short of the winning post. Indeed, it is quite possible that Watkins will come third on Thursday. Labour's Debbie Abrahams is what the bookies call "nailed on" to win, and the Conservative, Kashif Ali, could be the runner-up.

My prediction has little to do with the qualities of the candidates or their campaigns. Provided the candidate is not an obvious liability he or she does not make much difference in a modern by-election campaign. I was reminded by a veteran by-election watcher, who was in Oldham last week, of Bob Gillespie, Labour's candidate in the Glasgow by-election in 1988, who had LOVE and HATE tattooed on his knuckles. But by-election voting is mostly affected by two main factors.

The baseline is provided by national opinion polls. In this case, Labour is up 10 points since the general election, the Lib Dems down 14, and the Conservatives unchanged.

Then there is the anti-politics factor. By-elections have long been a means of expressing not merely hostility towards the Government, but towards the established order generally. Hence that staple of British politics, the Liberal, Alliance or Liberal Democrat by-election "upset". The rules of that game have changed. Nick Clegg is now The Most Hated Man in Britain, and only his outstanding unpopularity shields the Conservative wing of the coalition from greater contagion. Thus, in English seats and subject to the Gillespie proviso, the Labour Party is by default the beneficiary of anti-politics sentiment. Labour may be part of the establishment, but the option of a third party untainted by the responsibilities of office is no longer available. It is as if Labour were Her Majesty's Opposition and a protest party rolled into one.

Satisfying as it will be for Labour to see Clegg fall victim to the anti-politics wave that he used to ride against them so lazily, it would be a mistake to see victory in Oldham as a new dawn. Winning a by-election is no substitute for credibility on managing the economy, which is what the general election will be about.

That is why Ed Miliband was misguided in his broadcast with Jeremy Vine on Radio 2 last week. It was not that he seemed discomfited by the relentless hostility of the callers, although he should realise that the anti-politics mood is against him, too. It was not that he accused the Government of "deceit" about Labour's role in the fiscal crisis, although it is not a word calculated to win over those who think all politicians are liars. It was that he denied that Labour had made the crisis worse than it need have been.

If he persists in this, the sceptical centrist voter will not listen to the sensible part of his argument. The SCV knows that Gordon Brown increased public spending during a boom. He did it while he was prime minister, and Tony Blair's memoir records that he did it before he became prime minister, possibly with a view to an early general election in 2008. Blair announced a Fundamental Savings Review in February 2006 to restrain the growth of public spending, but it was "fought every inch of the way" by Brown, abetted by Ed Miliband, who was at the time struggling to fill Ed Balls's shoes as the Chancellor's economic adviser.

As Philip Collins, one of Blair's former advisers, put it last week, the accusation against Ed Miliband "is not that spending 'caused' the deficit, but that he was driving too fast, with no insurance, when there was a crash".

Miliband needs a better line, one that has more credibility by virtue of being true. Which would be that he was sorry that Labour continued to spend too much (probably better not to add, "or tax too little") when it would have been prudent to provide a cushion for an adverse turn in the global economy. Having thus engaged the listener's interest, he could go on to point out (as he did last week, to no effect) that the Conservatives supported Labour in making this mistake every step of the way (until the crunch actually came, when David Cameron and George Osborne started to make all the wrong decisions about how to respond to the crisis).

The danger is that Labour's victory in Oldham will encourage the party to misread an anti-politics spasm for winning an argument, when the voters are simply not listening.;