'The kaleidoscope has been shaken," one Labour strategist told me this weekend, echoing Tony Blair's words after the September 2001 attacks in America. Then he added "a bit", just in case I got too carried away with Nick Clegg's triumph in the leaders' first televised debate.

Labour wants the Liberal Democrats to do well. But not too well. If the third party does respectably, it will stop the Tories capturing their seats in the south-west and south, which would suit Labour very nicely. But if the Lib Dems really take off, they threaten Labour in its northern heartlands.

A week ago, transport secretary Andrew Adonis wooed Lib Dem supporters in an article in which he offered them a rather one-sided pact: 'vote for us to keep the Tories out'. On Friday, Adonis was asked on BBC Radio 4 whether he would serve under Clegg. The terms of political trade have been transformed by 90 minutes of live TV.

No one knows what happens next. This is a moment of genuine excitement and unpredictability. It is not a normal election, not because of the economic crisis but because the MPs' expenses crisis has created a much bigger pool of undecided and disenchanted voters. The "don't knows" were at 15% in the most recent ComRes survey. If Clegg can scoop many of them up, he really will be in business.

There is a frenzy in the Westminster village. The ComRes website crashed as word spread of a poll showing the Liberal Democrats only one point behind the Tories with Labour a poor third. It turned out to be less dramatic when the figures, from 4,000 people who watched the debate, were extrapolated into national share of the vote, which showed the Tories seven points ahead of Labour.

Despite that, there were distinct echoes of the rise of the SDP-Liberal Alliance in the 1980s, the last spell of three-party politics in Britain. The third force was briefly well ahead in the polls and almost broke the mould by pushing Labour into third place at the 1983 election.

The current wave of excitement may prove to be another false dawn. The Tories and their media allies will put the Liberal Democrats under intense scrutiny. Although Clegg has pruned the "money grows on trees" tendency in his party, he has not entirely uprooted it. The pages at the back of his manifesto, with columns of spending totals, read like a budget. It looks credible, and not very Liberal Democrat. But there are still some weak points, such as a magical £4.6 billion saving from clamping down on tax avoidance. Tory officials will pore over the fine print this weekend; we will see the results next week.

The key issue now will be whether the Tories and Labour can persuade voters it is still a two-horse race. The biggest obstacle in the third party's path has always been the "wasted vote" argument, the Catch 22 of British politics: many more people would vote for them if they thought they could win but they don't and so they don't vote for them. The TV debates will dilute this, but don't bank on it – nor underestimate the determination of the big two parties to preserve their dominance.

While professing to be relaxed, the Tories are privately disappointed that the "story" of the campaign is not Cameron's unstoppable march to power. That has been interrupted, perhaps only temporarily, but the Tories will anxiously await the next crop of opinion polls.

For me, the TV debate illustrated the limitations of the "big society" theme at the heart of the Tory manifesto – replacing Labour's "big government" by handing over public services to voluntary groups, charities or small groups of residents. Cameron believes passionately in it. It is not a secret plan for cuts or a return to Victorian values, as critics maintain. But it is vague – fine as a plan to be implemented after winning power but not much use on the doorstep as you try to win it.

Some Tories are saying "I told you so" and will be demanding a harder edge – raising the prospect of "five more years of Brown" and policies such as law and order.

Some critics will urge Cameron to turn up the volume on immigration, but I doubt he will do that. He believes the public is fully aware of his party's stance on the issue and is still scarred by the 2005 election, when it ran out of control when the then-leader Michael Howard played the immigration card, reviving the "nasty party" label.

Labour hopes the Clegg bandwagon will halt Cameron's advance, without overtaking Brown in the process. Well away from the TV cameras, senior Labour and Liberal Democrats play footsie, knowing their parties have much in common. The immediate, instinctive reaction to the Tory manifesto launch by Clegg and Peter Mandelson was identical, even though they didn't speak. However, talking in public about post-election co-operation would play into the Tories' hands, allowing Cameron to say a vote for Clegg is a vote for Brown.

By halting Cameron's bandwagon, Nick Clegg has given Gordon Brown a second chance, another opportunity to convince voters to stick with the "devil they know" rather than the two they don't. We will soon know whether Brown can take it.

Second Debate: Sky News, Bristol, Thursday 22 April, 8pm

Presented by: Adam Boulton

Topic: International affairs – Afghanistan, Iraq, Iran, Middle East, China, defence, international terrorism, the EU, climate change

Third debate: BBC One Midlands, Thursday 29 April, 8.30pm

Presented by: David Dimbleby

Topic: Economy – financing of public services, tax, debt, fiscal policy, recession, recovery, banking and finance, business, pensions, jobs