Prime Minister Gordon Brown and Conservative chief David Cameron scrambled to seize back the initiative from the Lib Dems, who could hold the balance of power if no party secures a majority.
A series of opinion surveys put the Lib Dems in second or even first place after their leader Nick Clegg's powerful performance in a first-ever TV debate last week with his Labour and Conservative counterparts.
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Brown, battling to avoid an end to 13 years of Labour rule, admitted he had lost the April 15 debate against Clegg and Cameron, at least in terms of presentation.
"I think it's energised the campaign. It's thrown the campaign wide, wide open. People thought it was a closed book to start with. I lost on presentation. I lost on style," he told BBC television.
But he insisted: "This isn't a sprint... This isn't an X-Factor talent show," adding: "I've learned at the end of the debate, substance will come through."
Televised debate
Snap polls after last week's debate showed Clegg the clear winner, and a poll the following day had his Lib Dems up three points at 24 per cent in overall voting intentions, although still in third place.
But polls in Sunday newspapers showed an even stronger surge: a BPIX poll in The Mail on Sunday even put them in first place on 32 per cent, against 31 per cent for the Tories and 28 per cent for Labour.
Three other polls put the Lib Dems in second place, behind the Conservatives.
If sustained, such figures make a hung parliament and a rare coalition government seem even more likely.
Under Britain's electoral system, such a result could see Brown's party end up in pole position - despite coming third in the popular vote.
The BBC's seat calculator said those figures would still put Labour on 276 seats - 50 short of a majority, the Conservatives on 226 and the Lib Dems on 119.
Swinging voters
The main opposition Conservatives - leading in the polls for more than two years - now face a major fight to stop the crucial swinging voters they need from switching support to the Lib Dems.
Clegg said "something exciting is happening" in the British political system, which has seen the Conservatives and Labour trade power for the best part of the past 100 years.
But he dismissed as "completely absurd" a suggestion - reported in a Sunday Times front page story - that he was more popular than British wartime leader Winston Churchill.
Bob Worcester, founder of pollsters MORI, wrote in The Observer newspaper: "Up to now this election has been static; now it's electric.
"Before last Thursday's debate, the Liberal Democrats were a sideshow; now they are centre stage."
