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, coming 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 May 6 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.

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.

The shape of politics will be transformed on May 6. 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 the Tories would be the largest party in a hung parliament - which is where the country was before the Cleggshell was dropped on this campaign.

The voters 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.

There are all manner of baubles a Tory government could offer the Lib Dems without going to the trouble of a formal coalition. The big question is whether Cameron concedes any change in the voting system - the holy grail of third-party politics.

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. 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 party in 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?

Clegg told the Independent last week: "We think AV-plus is a feasible way to proceed. 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 Alternative Vote on its own, and it was the first time Clegg had offered a compromise electoral reform that has a Labour rationale.

The other way might take longer. If Cameron is prime minister 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.