John Rentoul: The shape of things to come

As the Lib Dems hold their ground, the Cameron and Brown camps must determine the price of victory

The really surprising thing about the Nick Clegg surge is that almost nothing has changed. That may seem an odd thing to say after 10 days in which Labour has been forced into third place in the opinion polls and the Liberal Democrats have broken through for the first time since the formation of the Social Democratic Party nearly three decades ago. And this surge is better timed, in that it comes in the middle of an election campaign. Indeed, it is likely that, unlike the SDP/Liberal Alliance, the Lib Dems will win more votes on 6 May than the Labour Party.

But the way votes translate into seats means that, unless the Lib Dems get up to 36 or 37 per cent of the vote, they remain the third party in seats. And while Clegg's party remains the third party in the House of Commons, the outcome of the election is decided by the gap between the Conservatives and Labour. It was fun while it lasted, and the cult of Clegg is not over, but it is on a plateau. I doubt that the Lib Dems will continue to gain support over the last few days of the campaign. Last week's debate was his equivalent of Labour's 2001 general election – a second "landslide" that left the landscape unaltered. For those of us who can remember the days of air travel, cruising at altitude is a lot less interesting than taking off.

Equally, though, there is no reason why the inevitable cycle of boredom, disappointment and betrayal should start in the next 11 days. The shape of politics will be transformed on 6 May. That may be the beginning of the end for the Labour Party. And yet the outcome of the election remains surprisingly unchanged.

The reason is that Clegg's surge has been uncannily even-handed in its effect on the other two parties. The Conservatives have fallen 4.5 points in the polls, on average, since the first debate, and the Labour drop has been the same. The gap between the two is therefore unchanged, at about 6.5 points, which suggests that the Tories would be the largest party in a hung parliament – which is where we were before the Cleggshell was dropped on this campaign.

We are likely to end up, therefore, with David Cameron as prime minister, leader of a minority Conservative government. Clegg has already said that he respects the right of the party with the most votes or the most seats to try to form a government. Cameron will no doubt offer senior Liberal Democrats places on a national council of economic brassplate, which has been proposed by Clegg.

If you study our magnificent Venn diagram of party policies on pages 52-53, you will see that there are all manner of baubles that a Tory government could offer the Lib Dems without going to the trouble of joining them in a formal coalition. The big question is whether Cameron concedes any change in the voting system – the holy grail of third-party politics. And the big answer is no.

Should the Tories win the most votes, Cameron would have legitimacy to rule. It wouldn't be comfortable and may not last, but it would be accepted.

To go further down this hypothetical route risks the wrath of those such as Alastair Campbell, who complained last week that election coverage was becoming obsessed with "hung parliament processology". Too bad. I intend to learn from my mistakes, and the mistake we all made in failing to predict the Clegg breakout was in not pursuing hypothetical scenarios.

There are two ways in which the Lib Dem advance could change our political system for good. One would be if Gordon Brown succeeded in pulling back to the point – not far off if the polls are right – where Labour is the largest partyin parliament. One thing that unites the commentators in this election is a respect bordering on awe for Brown's resilience and tactical skill. He has forced the Conservatives into a position where, every time they are asked a question, they change their policy. Before the campaign, they ditched fiscal responsibility to promise lower National Insurance contributions. Once their manifesto had gone to the printers, they announced a pay limit for the public sector and the details of the marriage tax allowance. In last week's leaders' debate, Cameron spent hundreds of millions of pounds in a second when he said the Tories would keep free eye tests for old people.

If Brown emerged as leader of the largest party, it would likely be on the basis of coming third in votes. Then, he would have to pay a heavy price for Clegg's permission to remain in office. But who believes that there is a limit to what he would offer to stay in power? There has been some private excitement in the anti-Brown faction of the Labour Party recently that Clegg might succeed where Patricia Hewitt and Geoff Hoon failed. But that is another coup that has fizzled out before it has even begun. In what may turn out to be one of the most significant interviews of this campaign, Clegg named his price in The Independent last week. "We think AV-plus is a feasible way to proceed," he said. "At least it is proportional – and it retains a constituency link."

Well, AV-plus is not truly proportional, but it is a lot more so than Brown's favoured option of AV on its own, and it was the first time Clegg had offered a compromise electoral reform that has a Labour rationale: it was recommended by Roy Jenkins as a way of fudging Tony Blair's promise of a referendum on the subject in the "new dawn".

The other way might take longer. If Cameron is PM in a hung parliament, the situation is inherently unstable: he would be looking for a way to dash to the polls for a second election to secure a majority. But he may be forced to deal, and there are rumours that he once said that, if there had to be change, it should be to a system that is fully proportional.

The Clegg surge may not change the outcome of this election, but it may change all elections in future.

John Rentoul blogs at www.independent.co.uk/jrentoul