Nick Clegg has condemned the "preposterous" idea that either of the main parties could govern alone if they fail to win an outright majority in the general election.
The Deputy Prime Minister said calls for the Tories and Labour to rule out a coalition with the Liberal Democrats next year were the "last gasp" of "tribal" elements.
Unite general secretary Len McCluskey has urged Ed Miliband to say he would lead a minority administration even if the party falls short of complete victory in next year's general election.
The comments came amid speculation that David Cameron could try to galvanise the Conservative rank and file by making the same promise in the run-up to the poll.
Speaking at his regular press conference, Mr Clegg said: "Clearly there is a sort of McCluskey tendency in both the Labour and Conservative parties.
" I think what you are seeing, in a sense, is the last gasp of the assumption from the two bigger parties that somehow they have got a right to run things.
"They are now so desperate they are now somehow claiming that they will have the right to decide how this country is governed even if they don't win a majority, which is a clearly preposterous assertion.
"We should let the British people have their say, rather than people constantly assuming that they can decide rather than the British people about how this country is governed."
The Lib Dem leader said those elements were "taking the British people for granted", adding: "I think what you are seeing is some very tribal voices who are so tribal that they want to even go so far as deprive the British people of stable government.
"Because minority government is deeply unstable, because you are basically governing without the authority to do so...
"I think a Labour-only government would jeopardise the economic recovery and a Conservative government certainly would not govern with the emphasis on fairness that has been the hallmark of much of what the Lib Dems have brought in the coalition."
Mr Clegg said the Prime Minister had not indicated to him that he was planning on ruling out an alliance with the Lib Dems after the general election.
He also denied that the coalition was running out of steam and descending into infighting with more than a year to go until the poll.
The public understand that people do not always have to agree on everything or get on personally to work together effectively, he said.
In an interview with the BBC's Newsnight programme, Mr McCluskey said: "Labour, I hope, win the next election outright, but if they are the biggest party, then my view is Ed should have the courage of his convictions and govern on a minority government...
"My view is that Ed shouldn't be sucked into a Lib/Lab pact. He should have the courage of his convictions - if we are the largest party, he should govern.
"And he should challenge those coalition parties to bring him down if necessary and go back to the people so that there's a stark alternative."
Labour and Mr Clegg have been seen as positioning for a potential alliance next year.
Ed Balls and Mr Clegg have buried the hatchet after a long-running feud, and the Deputy Prime Minister recently praised the way the Opposition had "changed" under Mr Miliband.
A decision by either main party to rule out a coalition would aim to put the choice for voters into stark relief.
But it would also cause a major strategic headache for the Lib Dems, who are highlighting their ability to temper the excesses of the others as a main selling point.