BRISTOL, England (Reuters) - The two main candidates to lead Britain after a May 6 election sought on Thursday to fend off a surprise challenge from a smaller party in a lively TV debate, but there was no undisputed winner.

Britain's Liberal Democrat leader Nick Clegg listens to members of the Infant, Mother and Pre School group at the Community Church in Greenway during an election campaign visit to Bristol, western England April 22, 2010. The Liberal Democrats have doubled their projected vote share in Labour-held marginal constituencies but the swing comes mainly from voters who were previously not sure they would vote, a poll showed on Thursday. REUTERS/Gareth Fuller/Pool

Frontrunner David Cameron of the center-right opposition Conservatives and Prime Minister Gordon Brown of the Labour Party were under pressure to halt the rise of Liberal Democrat leader Nick Clegg after he outshone them in an earlier debate.

“An awful lot of fire was turned on Clegg, and I think they got him on the ropes, but not on the canvas,” Jon Tonge, politics professor at Liverpool University, told Reuters.

Thursday’s clash, on issues from Afghanistan to Europe and pensions to immigration, was the second of a series of three planned for the campaign -- the first time in British history that the main party leaders have taken part in TV debates.

Two snap polls after Thursday’s contest gave conflicting results, though both suggested that the performances were more balanced than a week ago when Clegg was the clear winner.

A YouGov poll for the Sun newspaper put Cameron in first place with 36 percent, Clegg in second with 32 percent, and Brown last with 29 percent. A ComRes poll for ITV News put Clegg ahead with 33 percent, with Brown and Cameron both on 30.

Opinion polls on the election itself, which until a few months ago pointed to a Conservative win, have in recent weeks suggested that no party may win an overall majority. That could hand the balance of power to the previously little-known Clegg.

His star turn in the first debate led to a surge in the popularity of the Liberal Democrats, a previously ignored centrist party presented by Clegg as “real change” from the two big parties, who have alternated in power since World War Two.

“Something really exciting is beginning to happen. People are beginning to believe, beginning to hope, that we can do something different this time,” he said in his closing remarks.

CAMERON COMEBACK

Cameron, who also has campaigned on the theme of change after 13 years of Labour government, mounted a strong fightback. Criticized for being unusually stiff in the first debate, he regained his usual fluency and telegenic style in round two.

He argued that far from being different, the Liberal Democrats had been tainted just like Labour and the Conservatives by a huge scandal last year over legislators’ dubious expense claims that infuriated millions of voters.

“Frankly, Nick, we all had problems with this ... Don’t anyone try and put themselves on a pedestal,” he said.

The Conservatives say that if they do not win a clear victory on May 6, squabbling between the parties to form a ruling coalition will delay tough decisions on cutting a worrisome budget deficit and leave Britain weak and divided.

They fear that Labour could hang on to power with support from the Lib Dems. They have tried to halt Clegg’s rise with the slogan “Vote Clegg, Get Brown.”

Cameron’s improved performance will be a huge relief to the Conservatives after a week during which the campaign has been dominated by talk of Clegg’s sudden ascendancy.

Brown, who is the first to admit presentational skills are not his strength, gave the most awkward performance of the three but tried to present himself as the strongest on substance.

“If it is all about style and PR, count me out. If it’s about the big decisions, if it’s about judgment, if it’s about delivering a better future for this country, I’m your man,” he said in his opening statement.