But he said the campaign was not "an X Factor talent show" and people were now focusing on which party and leader could deliver on the big issues such as the economy, the NHS and education.
"The Liberal Democrats have got to be exposed," he said.
"I think they have made a mistake in their economic policy...Why do they want to cut child tax credits? I think that is unfair. Why do they want to cut child trust funds?"
Business Secretary Lord Mandelson reinforced the attack, saying the Lib Dems could hold "disproportionate power" in a hung Parliament and make it difficult to pass counter-terrorism legislation.
"My bet is that most people will not follow through on their current flirtation with Nick [Clegg]", he said in a memo to activists.
Labour also attacked Conservative plans to cut £6bn in spending this year, which they say would hit jobs but which the Tories say is needed to stop the planned National Insurance rise.
"People will start to say there is a risk here and it is too big a risk on the economy to take," Mr Brown said of the Tory plans.
Earlier, he told the Sunday Telegraph that the prospect of a first Conservative budget - which the Tories have said they would hold within 50 days of being elected - sent "a shiver down my spine".
"People know economic security is an issue. It is on the ballot paper," Mr Brown told Andrew Marr.
The Liberal Democrats reported a leap in donations following Thursday's debate.
Mr Clegg told an audience of young people in south London that although there was a long way to go in the campaign and polls were "volatile", the election represented an "immense opportunity" for change.
"A growing number of people are starting to hope, to believe a little door has opened and that maybe this time we can do things differently," he said.
If you want real change...the only way is to have a Conservative government
Mr Clegg accused the Conservatives of "recycling misleading claims" about its European policy, after they said the Lib Dems were ready to sign up to anything proposed by the EU to give away British powers.
Outlining his agenda for older people in Swindon, Mr Cameron said the only way the public could be "absolutely certain" of getting new leadership after the election was by voting for the Conservatives.
Refraining from mentioning Mr Clegg, he said: "If you want real change, if you want the job to be done, the only way is to have a Conservative government."
The BBC's Political Correspondent Carole Walker said Mr Cameron wanted to concentrate on the "real choice" in front of the public, which he saw as a straight fight between him and Gordon Brown.
The apparent swing in the polls follows the first of three prime ministerial TV debates on Thursday, in which Mr Clegg was acknowledged by his rivals to have shone.
We have no historical comparisons to help us judge whether this turbulence will now dominate the campaign
David Cowling, Editor of the BBC's political research unit
The editor of the BBC's political research unit, David Cowling, said what the ComRes poll appeared to confirm was the "extraordinary shift in Lib Dem support as a result of the first prime ministerial debate".
However with two more debates to follow, he warned it was difficult to say if the swing would translate into votes on polling day.
When the results of the ComRes poll of 1,006 people - conducted on 16 and 17 April - were fed into the BBC's Commons seat calculator, it showed Labour would still have the most seats with 279, the Conservatives 239, the Lib Dems 103 and others 29.
Meanwhile, an ICM poll for the Sunday Telegraph - conducted largely before Thursday's debate - puts the Conservatives on 34%, Labour on 29% and the Lib Dems on 27%.
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