LONDON (AP) — An unpredictable British national election has suddenly become more uncertain, with an unexpectedly stellar television debate performance from the leader of the Liberal Democrats sending the perenially third-ranked party to first place in some opinion polls.

Nick Clegg's party has leapfrogged the main opposition Conservatives — who most had expected to oust Prime Minister Gordon Brown's governing Labour after 13 years in office. The Liberal Democrats' success is overturning all previous predictions for Britain's May 6 poll.

Clegg's relaxed style and sharp attacks on Britain's two major parties won him, by virtually every asseessment, the country's first ever TV debate last week, and prompted an opinion poll surge that's eclipsed his rivals.

The 43-year-old Clegg attacked Brown and Conservative leader David Cameron as part of an old consensus in British politics that allowed a lawmakers' expense scandal and failed to anticipate the financial meltdown.

He also charmed viewers by referring to audience members who asked questions by name, in a style reminiscent of President Bill Clinton's.

"The more they attack each other, the more they sound the same," Clegg said of his two major rivals as the three leaders debated Thursday, watched by a TV audience of 9 million viewers.

An opinion poll by BPIX, published Sunday, put the Liberal Democrats up 12 percentage points to 32 percent. Cameron's Conservatives fell seven points to 31 percent and Brown's Labour sat third with 28 percent. The survey of 2,149 people had a margin of error of plus or minus two percent. In a poll Monday by ICM for The Guardian newspaper, the Liberal Democrats were up 10 points to 30 percent — behind the Conservatives, but ahead of Brown's Labour.

Several other polls since the TV debate — the first of three in the campaign — have also put Clegg's party in the lead.

"The general election campaign is starting to come to life," Clegg said Monday, in Cardiff, Wales. "A growing number of people are starting — and it is only a start — to believe, starting to hope, that we can do something different this time."

Clegg's Liberal Democrats, who last held power as the Liberal Party in the 1920s, are on course for their best national election result since the merging with the Social Democratic Party in 1988.

The party — which usually takes about 20 percent of British votes — is fiscally conservative but socially liberal. It opposed the Iraq war, says it won't support strikes against Iran if diplomacy fails over its disputed nuclear program, and is uneasy at a rising death toll in Afghanistan.

"It's an unprecedented surge in an election campaign," said Anthony Wells, an analyst who runs the UK Polling Report website. He said the new surveys suggest Clegg's bounce could be decisive in ensuring that both Labour and the Conservatives are denied an outright majority on election day.

And it means the real winner from Clegg's surge could be Brown, who — thanks to Britain's quirky election system — is set to win most House of Commons seats and likely remain as British leader in the country's first hung Parliament since 1974.

Labour traditionally wins more seats than its rivals even when it has a lower share of the total number of votes — mainly because the party has more backing in urban areas, where Parliamentary districts have fewer voters and usually have a lower turnout than in rural areas.

In order to oust the 59-year-old Brown, Cameron is also seeking to snatch a number of key seats from the Liberal Democrats. The more of those seats a stronger Liberal Democrats can hold, the more likely Brown can cling to office.

An analysis of latest polls by Wells predicted that in the 650-seat House of Commons, Brown's Labour would win about 250 seats, Cameron's Conservatives 240 and Clegg's Liberal Democrats 130. It would leave both Labour and the Conservatives far short of the 326 seats needed to command an outright majority, but would allow Brown to lead a minority government — or try to strike a pact with Clegg.

In return for his support, Clegg will likely demand major reforms to Britain's first-past-the-post voting system, insisting on a move toward proportional representation, under which allocation of Commons seats would more closely reflect the popular vote.

"Self-evidently, the electoral system is potty," Clegg said, referring to polls which indicate a third placed Labour Party would still claim the largest share of House of Commons seats.

Wells said that polling shows young voters and women, in particular, are switching their backing to Clegg's party since the TV debate.

"The big question now is what will David Cameron and Gordon Brown do to stop the great Liberal Democrat surge?" Wells said.

Speaking in London, Cameron promised not to resort to negative campaigning against Clegg — though his party has cautioned Britons that a vote for the Liberal Democrats could keep Brown in power.

Cameron, 43, said that in a hung Parliament "you are not going to get the decisive action and the change we need."

Economists have warned that if Britain's election fails to produce an outright winner, financial markets will react badly, fearing action to reduce the country's huge deficit could be delayed.

"Financial markets are always very averse to uncertainty," said Mark Ostwald, a strategist at Monument Securities.

Brown insisted he is campaigning to win, but has acknowledged he and Clegg agree over several policy areas, including the need for electoral reform — meaning a pact in a hung Parliament would be possible.

But Brown, who enjoyed a brief surge in opinion polls after replacing Tony Blair in June 2007, warned Clegg not to expect his newfound popularity to last.

"I know a little about what it means to have a short political honeymoon," Brown told reporters Monday at a news conference in London.