Nick Clegg delivered a thinly-veiled warning to David Cameron today that he must face down a major Tory rebellion over Lords reform.
The deputy prime minister pointed out that he had asked Liberal Democrats to support coalition measures they did not like, and others should act in the same "spirit".
However, he also hinted at possible concessions to placate opponents - even avoiding ruling out a referendum.
The comments, on the BBC's Sunday Politics show, came amid reports that at least six Cabinet ministers would prefer to see the changes delayed.
Philip Hammond, Iain Duncan Smith, Michael Gove, Eric Pickles, Owen Paterson and Lord Strathclyde are said to be among senior Conservatives who have voiced doubts.
The party's backbenchers are also threatening a mass revolt on the issue, with the potential for resignations by several ministerial aides.
Earlier today, Justice Secretary Ken Clarke - an ardent backer of Lords reform - admitted that a purely Tory government would not be pushing it through in this parliament.
Speaking on Sky News' Murnaghan programme earlier, Clarke said: "The existing House of Lords is a curious historical anomaly.
"We are ready for democracy, I think. All three political parties were in favour of House of Lords reform in their last manifestos."
But Clegg cautioned against a situation where the parties began blocking each others' favoured policies.
"I think one of the great things about this coalition, despite a lot of pressure to do otherwise, is that we haven't indulged in sort of tit-for-tat selective choice about which bit of the coalition agreement we are going to support or not," he said.
"We all entered into this government knowing first of all that no-one had won a majority... and secondly that we had a clear programme of reform we wanted to introduce and we back them.
"I have asked Liberal Democrat MPs and peers to back a number of things - the NHS Bill, other things - that they didn't like at all.
"But I did it because it was in the spirit of the coalition, and I would ask all people from all sides of the coalition Government to continue to govern in that spirit because it is what I think the British people want."