As he was driven in a Special Branch car to Manchester at 6.30pm on Thursday evening, the last phone call Nick Clegg made before arriving at the ITV studios was to his three young boys. There was no mention during the conversation of the big event in dad's diary. Antonio, 9, was bothered about his new watch while Alberto, 5, was nagging his father to get on with planning his birthday party.

"They were not really aware," said an aide. "Nick agreed with Alberto that the birthday arrangements really were the most urgent thing on the calendar for the next few weeks."

Keeping everything in perspective was vital. So was mental and physical preparation. Clegg had rejected the idea of staying in a Manchester hotel with all the other political groupies. "I don't want all that air conditioning. I want some real air," he told his team. So for the night before the first TV leaders' debate in British electoral history, they checked him into Shrigley Hall, in rolling countryside near Macclesfield. Before setting off for the TV studios, Clegg went for a long stroll on his own in the grounds and surrounding hills to clear his head and calm his nerves.

It worked. A few hours later, as he stepped off stage after 90 minutes of argument with Gordon Brown and David Cameron in front of almost 10 million viewers, the Liberal Democrat leader rang his wife Miriam. She told him he had been "brilliant". Then his chief of staff, Danny Alexander, made contact to say that, according to the instant polls, much of the nation agreed. Clegg had finally emerged from the shadow of his colleague Vince Cable, who until then had hogged all the limelight.

The next morning, as the Lib Dems tried to come to terms with a media that had, overnight, recast their leader from insipid also-ran to hero, poll results that Clegg could not have dreamed of 24 hours earlier were still pouring out.

By then, the Conservative battle bus, with David Cameron on board, was heading disconsolately for Wales. Tory MPs and officials were struggling to comprehend a whirlwind that threatened to blow previous assumptions about the 2010 election off course.

At one point, the findings of an ITV-ComRes poll flashed up on one of the bus's TV screens. It claimed the Lib Dems had leapt 14 points to 35%. "Bloody hell," said one Tory aide under her breath, before other officials dismissed the poll as misleading. On the mothers' internet forum Mumsnet, 44% of women who voted in a post-debate survey said they were now thinking of voting Lib Dem, compared with 23% three weeks ago. The Lib Dems' following on Facebook doubled in a day.

Tory aides tried not to look deflated as they admitted Clegg had enjoyed a marvellous night. They promised to stick to their strategy, implying that they would not divert the Tory attack dogs from Gordon Brown on to the Lib Dem leader. But within the hour they had released an 11-point dossier that suggested the reverse was the case. Entitled "Deconstructing Nick Clegg", the email attacked the Lib Dem's claims on schools, expenses, Trident, immigration and more.

After the Tory bus left Prestatyn in Wales, it headed to an Asda in Wolverhampton. There, Louise Anyon, a 22-year-old trainee manager, asked Cameron about pensions. After he left, she said she had been thinking of voting Tory but was now wobbling. "Nick Clegg was so brilliant," she said, beaming. "I'm very tempted by the Liberal Democrats."

Tories cannot allow the Lib Dem leader to prolong his time in the sun if they can help it. They will attack him over Europe – Clegg used to advocate entry into the eurozone and is a former European Commission official – and immigration. And the right wing of the Tory party is restless for Cameron to go back to basics. They desperately want to reverse the situation has emerged, with polls today placing the Liberal Democrats in second place. Strange though it seems, if today's polling was replicated on May 6 that would result in a situtation in which Labour would have more MPs than any other party. "The Conservative forces of hell will be unleashed on us," predicted Olly Grender, former communications director to Paddy Ashdown.

But despite the inevitable onslaughts, most Lib Dems believe they have a real chance to break the mould of two-party dominance in British politics. A senior Lib Dem strategist said the new forum of a TV debate was a factor that really could change the game. "You have to understand that Lib Dem leaders don't get a honeymoon with the electorate," the source said. "They hardly even get a wedding night. They don't have that period of exposure. Like Ashdown and Kennedy, they get elected then are either ignored or hounded.

"I think Nick found that frustrating. And because he got so few opportunities to be listened to on serious subjects, when he did get on the media he sometimes tried to say 50 things in a minute. But now that he's on an equal footing, he is showing what he can do."

Another factor, they say, is that this is no ordinary election. Huge numbers of voters are disillusioned with politicians following the expenses scandal. That disgust with politics has made voters less inclined to adopt the "Buggins's turn" principle of switching support between Labour and the Tories. Because Clegg was, before Thursday, little known to most voters, the Lib Dems are confident he is also largely untainted.

Chris Huhne, the Lib Dem home affairs spokesman, said people were thinking in new ways about how to vote, some even saying that they favoured a hung parliament: "I've never before had people saying they don't want one party in overall control. That is extraordinary and completely new."

Alexander argues that the election has been thrown completely open. "What is going on is that there is a group of people who endorsed Tony Blair in 1997 who we are all fighting over – the Tories, Labour and us. But what has changed since Nick's TV performance is that those who were thinking of holding their noses and voting for David Cameron will now be thinking of voting for us. And those who were thinking of holding their noses and voting for Gordon Brown will be doing the same."

By giving the Liberal Democrats equal billing with Labour and the Tories, the three leaders' debates have added to the unpredictability of an election that, pollsters agree, was already proving the most difficult to predict in decades. They have given Clegg the chance to pose as the new advocate of change, marshalling messages on the need for reform of the voting system and the House of Lords that they believe will make Cameron look stuck in the mud.

It is the undecided voters that Clegg's team now have in their sights; the last ComRes poll had the "don't knows" at 15%. On Friday night, that volatility was evident when a YouGov survey found the Lib Dems had climbed eight points since the debate to 30%, overtaking Labour, who were down three at 28%. The Tories had slumped by four to 33%.

The Lib Dems were not only buoyed by the raw data: they also felt it greatly strengthened their argument for a move to fairer voting. Due to the vagaries of the electoral system, if YouGov's findings were replicated on election day, Labour, though in third place, would still have the greatest number of MPs: 276, against 246 for the Tories and 99 for the Lib Dems. "That is totally scandalous," said one of Clegg's team.

Despite the weighting of the voting system against them, the implications of a continued poll surge for the Lib Dems are huge – particularly for David Cameron's ambitions. Experts say that the boost for Clegg's party makes it more likely to hold on to seats in the south-west that the Tories have been targeting hard. But it may not be enough to make inroads in the Labour-Lib Dem marginals of the north. In fact, today's ComRes poll for the Independent on Sunday and Sunday Mirror, which places the Tories on 31, Lib Dems on 29 and Labour on 27, translates into a hung parliament scenario in which it is Labour that has most seats. It is the lowest level of support for Cameron's Conservatives since Brown's honeymoon as Labour leader in 2007. Perhaps that is why the prime minister patted Clegg warmly on his arm as they walked off stage: Labour don't need to panic about the Lib Dem surge. As long as it does not turn into a charge it is welcome.

Even before the TV clash, the challenge the Conservatives faced was huge: they needed to hold all their existing seats and win 116 more to gain an overall majority. Having plastered the country with posters attacking Gordon Brown, they now have a new enemy.

Like Cameron, he is young, articulate and claims to represent change. The problem for the Conservatives is that a large slice of the country appears to have warmed to him with less than three weeks to go until polling day. Cameron, by contrast, was at the height of his popularity in the summer of 2008, when the Tories swept to victory in the traditional Labour heartland of Crewe and Nantwich.

"We have to keep this in check," said one senior Lib Dem. "But we have timed this one pretty well."

Now the Conservatives have to decide when and how they will attack the nation's favourite new leader. "If there is this wave of support for Clegg," said one Tory MP, "we can't be seen to be tearing into him too aggressively or too soon. But we need to pick apart his policies. It is a really difficult balance."

Liberal Democrat strategists believe one of the Tories' big mistakes has been to ignore the third-party threat at a national level for too long. "It is only those who are up against us locally that get it," said one. "The others think it is like Prime Minister's Questions: between Labour and the Tories. You get the impression people like Cameron think 'What's that? Who's that? What's a Liberal Democrat doing in the same room as us?' Now they are finding out."

On the campaign trail in south London yesterday, Clegg said people were beginning to realise that "something different" was possible. "I think that is what's starting. I can't predict what is going to happen in the election campaign, but I think something exciting is starting to happen."

Annette Brooke, who is defending Mid-Dorset and North Poole for the Lib Dems, said that the Tories were outspending her by some margin, but that the television debate had been worth far more to her party than a thousand Conservative party posters had been to her opponent.

"Before [the debate] started, I couldn't have anticipated that the outcome would be as brilliant as it was," she said. "I was nearly leaping off my seat. It's enthused our existing supporters and helpers, so that's given us a real boost. And, yes, we are meeting people who are changing their votes, too."