Ukip leader Nigel Farage has hinted he would be willing to "do a deal" with Labour if the terms were right.
In an interview with the New Statesman, Mr Farage declared: "I'd do a deal with the Devil if he got me what I wanted," when asked if he would enter into a coalition with Labour.
According to a survey this week by Tory peer Lord Ashcroft, Ukip is heading for a comfortable victory in the Rochester and Strood by-election with a 12% advantage over the Tories.
Mr Farage also suggested that if Mark Reckless, whose defection from the Conservatives to Ukip triggered the by-election, won, "other Conservative MPs might join us".
He claimed there are also Labour MPs considering a move, but they are holding back through "fear of the consequences".
In his Conservative Party conference speech, the Prime Minister argued that a vote for Ukip is really a vote for Labour.
David Cameron told his party that "you could go to bed with Nigel Farage, and wake up with Ed Miliband".
Asked if he would rule out any coalition, Mr Farage said: "Of course not. There is no left and right any more. Left and right's irrelevant."
He acknowledged that the prospect of any 'rainbow coalition' "sounds extremely unlikely", adding: "I can't see Ukip wilfully going into formal coalition with anybody."
But asked about confidence motions, Mr Farage remained open minded, responding: "I'd be very comfortable supporting anybody that gave me an opportunity to get my country back."
During the discussion, Mr Farage also spelt out his party's determination to erode Labour's traditional heartlands in the north and Wales, saying he was "coming after" Labour voters.
He said: "Everybody thought that people's tribal allegiance to Labour was as strong, if not stronger, than the tribal allegiance to the Conservative Party. What we're actually finding is, they don't even recognise the tribe."
He added: "We are now the only party that can challenge Labour in the north. The Lib Dems have gone. I think we can come second in every seat in the north of England.
"The question is: how many can we win? We're targeting our resources. We're going to target Heywood and Middleton pretty heavily. And Rotherham. And we're doing very, very well in Doncaster."
Labour leader Ed Miliband holds the seat of Doncaster North.