Britain's Liberal Democrats said on Sunday they would not back Gordon Brown in power if his party came third in the popular vote on May 6, even if the nation's quirky system gave Labour the most parliamentary seats.
"It's just preposterous the idea that if a party comes third in terms of the number of votes it still somehow has got the right to carry on squatting in Number 10 (Downing Street)," Lib Dem leader Nick Clegg told BBC TV.
Clegg blew open the election campaign two weeks ago with a strong performance in Britain's first U.S-style televised debate of party leaders, which saw support for his Lib Dems rocket, turning them from perennial also-rans into possible kingmakers.
Polls in Sunday newspapers suggest that the opposition Conservatives are set to become the largest party but will fail to win the number of seats needed to give them an overall majority, leaving a "hung parliament" with no clear winner.
The surveys also indicate that Prime Minister Gordon Brown's ruling centre-left Labour party are trailing Clegg's centrist Lib Dems, coming in third in public support.
Under Britain's first-part-the-post electoral system, Labour could remain the biggest party in the 650-seat parliament despite polling the third-largest number of votes.
Clegg spelt out for the first time that he would not back Brown to stay in government under those circumstances.
"I think a party which has come third, and so millions of people have decided to abandon them, has lost the election spectacularly (and) cannot then lay claim to providing the prime minister of this country," he said.
Instead, he vowed to support whichever party won both the most votes and the most seats, which current polls, though volatile, indicate is likely to be the Conservatives.
Britain has not had a "hung parliament" since the days of the global oil crisis in 1974 and the prospect has many worried urgently needed action to tackle a record budget deficit could be delayed under such a scenario.
Financial markets would likely be reassured if a coalition government was formed which had clear policies on tackling the deficit, running at more than 11 percent of GDP, analyst said.
The centre-right Conservatives want to make spending cuts more quickly and deeply than Labour.
IRRELEVANT
Labour and the Lib Dems are more natural bedfellows, and commentators say that senior figures in Brown's party have been making subtle public overtures to Clegg.
But in a newspaper interview published on Sunday, Clegg described Labour as "increasingly irrelevant" and the election was a straight fight between his party and the Conservatives.
The issue of electoral reform looms in this election and could be a factor spoiling the chances of a coalition between the Conservatives and Lib Dems, which have long campaigned for change, not least because it works against them.
Clegg said reform remained a top priority.
"I think it is more than that (non-negotiable). I think it is unavoidable for any party whatever the outcome," he said.
Conservative leader David Cameron has not ruled out change, but is a strong advocate of the existing system.
"I want us to keep the current system that enables you to throw a government out of office," he told the Observer newspaper.
With Labour's support appearing to ebb, Brown said he was still fighting for victory. Business Secretary Peter Mandelson warned party supporters not to switch to the Lib Dems.
"You might start flirting with Nick Clegg but that way you will end up marrying David Cameron," he told the Sunday Mirror newspaper. "If (people) vote Lib Dem, they are making it easier for the Tories (Conservatives) to get in."