Nick Clegg will launch a major campaign today to win the youth vote amid signs that young people are backing the Liberal Democrats in increasing numbers following last week's televised leaders' debate.

The Liberal Democrat leader, hoping to maintain the sudden surge for his party in the polls, will appeal to those aged 18 to 25 to register to vote before Tuesday's deadline.

The move is similar to that launched by Barack Obama during his campaign for the White House. Obama urged young Americans not to waste their vote and to join his campaign.

Clegg told the Observer: "I will be saying to young people that this is their chance to make a difference. It will be a message of hope, but also one of urgency. They can really affect this election campaign, but to do so they have to register."

Clegg, who appealed to the electorate during the debate to "do something different", saw his party leapfrog Labour into second, and cut the Tory lead to three points, in a YouGov poll yesterday.

A series of new polls out today are also impressive for the party and show a dramatic shrinking of the Tory lead. A ComRes poll for the Independent on Sunday and Sunday Mirror puts David Cameron's party on 31%, the Lib Dems on 29% and Labour on 27%. Two polls even suggest the party is now leading both Conservatives and Labour. Such an outcome would give Gordon Brown's party most seats in the Commons – perhaps explaining why Labour aides are more comfortable with the Lib Dem surge.

Campaigning in Bedford, Brown claimed to be the only leader of substance. "As the dust settles after the first two weeks, people will have all their views about style and about presentation," he said. "But an election like this comes down to big choices people have to make about the big issues of the future."

It comes as Cameron uses an article in today's Observer to outline his vision for a Big Society.

He reveals a Tory government would make all government data available to the public unless there is good reason for it to remain private. This plan would include releasing crime data on a street-by-street basis.

The Conservative leader was campaigning in his own constituency of Witney, Oxon, yesterday when he was berated by a former soldier. The man, accompanied by his young daughter, confronted Cameron as he canvassed voters . "I met you… years ago, my wife was pregnant with this one," he said. "Come back from Germany, nowhere to live, little one, just left the army – got back from Kosovo. You gave me your card and you said, 'Give me a ring and I'll help you out' – 30-odd phone calls and you didn't ring me back once."

Cameron said he was "very sorry about that, because I try to answer all the emails, all the messages", and asked if there was "anything I can do to help you now", but was told the situation had been resolved.

Cameron aides said the local party chairman had spoken to the man and was following up the case.

The party will spend this week hitting back at the Liberal Democrats. Poll data suggests the boost for Clegg's party is coming largely from the under-45s.

One poll gave Clegg's party a huge lead – 44 points – among those aged 18-34 compared with 28 for the Conservatives and 24 for Labour. Clegg knows that he will need to push for young people to register if he is to translate the support into votes on 6 May.

Figures issued last month by the Electoral Commission suggested that 56% of those under 25 and eligible to vote were not on the electoral roll.

A surge in the past couple of weeks, however, saw 250,000 people register online, of whom 40% were under 25. At the 2005 election, only 37% of young people voted.

All three parties spent the weekend reassessing strategy following the first of the three television debates. While Labour has seen its support fall after Brown came third in voters' polls following the debate, the party accused Cameron of running scared of a television interview with the BBC's most forceful interviewer, Jeremy Paxman. Clegg appeared on the Paxman slot last Monday and Brown has also agreed to appear. But Cameron aides said late yesterday that the Tory leader would take part in an interview.

This week Labour will try to turn the focus back on to the economy – the area judged most successful for Brown in Thursday's debate – before the release of official figures on unemployment and economic growth.