Nick Clegg hurriedly revised the Liberal Democrat post-election negotiating position today by insisting that he had not ruled out a possible deal with Labour in a hung parliament. However, he said that if Labour came third in share of the vote – with polls suggesting that is a distinct possibility – he did not believe that Gordon Brown could remain as prime minister.

His clarification marks a shift from the weekend when he appeared to suggest Labour would have forfeited the right to govern if it came third on 6 May. His remarks had alarmed some on the progressive left who argued that he was in danger of reducing the anti-Tory tactical vote.

Labour and the Conservatives condemned Clegg on the issue today. One cabinet minister said he was over-reaching himself and had become intoxicated with his own publicity, and the Tories said he was holding the country to ransom with his demands for electoral reform.

Clegg, however, has not been deterred from trying to set out the Lib Dem stance. He said: "I think, if Labour do come third in terms of the number of votes cast, then people would find it inexplicable that Gordon Brown himself could carry on as prime minister. As for who I'd work with, I've been very clear – much clearer than David Cameron and Gordon Brown – that I will work with anyone. I will work with a man from the moon, I don't care, with anyone who can deliver the greater fairness that I think people want."

Asked if he could work with the "man from the moon but not Gordon Brown", he said: "I just don't think the British people would accept that he could carry on as prime minister, which is what the convention of old politics dictates when, or rather if, he were to lose the election in such spectacular style."

Lib Dem officials confirmed that Clegg was singling out Brown as the man the country would not tolerate if Labour dropped to third in share of the vote.

Clegg's new formula raises the problem of how Labour could replace Brown, as well as the prospect of Labour having a second prime minister who has not won a mandate at a general election. Of likely successors, Alan Johnson, Ed Balls and David Miliband could all lay claim to take over from Brown.

Three days before the final TV debate, today's polls showed there had been no crumbling of Lib Dem support, which surged after the first broadcast. A Guardian/ICM poll put the Tories on 33%, the Lib Dems on 30% and Labour on 28% – the same as a week ago. A ComRes poll for ITN showed the Tories on 32%, down two, the Lib Dems on 31, up two, and Labour on 28, unchanged. Both polls suggest Labour could end up with more seats than either of the other two parties.

Today the campaign was again dominated by speculation over a hung parliament, with David Cameron claiming that Clegg was trying to hold the country to ransom by demanding electoral reform as a precondition for a coalition government. He reiterated that he was opposed to such a reform but without absolutely ruling out a referendum on the issue. It is possible Cameron could still offer Clegg a referendum on compromise reforms – somewhere between the status quo and the preferred Lib Dem option of the single transferable vote.

Cameron accused Clegg of wanting PR "so we have a permanent hung parliament, a permanent coalition, so we never have strong and authoritative and decisive government". He added: "It's now becoming clear he [Clegg] wants to hold the whole country to ransom, just to get what would benefit the Liberal Democrats."

The Tories switched their party election broadcast to say the choice was no longer between a majority Labour government and a majority Tory government, but between a majority Tory government and paralysis.

Douglas Alexander, the Labour election co-ordinator, said: "My sense is that Nick Clegg has somewhat overreached himself – maybe intoxicated by the publicity he has received – in getting into the prediction business."