Nick Clegg has delivered a thinly-veiled warning to David Cameron that he must face down a major Tory rebellion and press ahead with Lords reform.
The Deputy Prime Minister insisted Liberal Democrats had supported coalition measures they did not like, and others should act in the same "spirit".
However, he did hint at possible concessions to placate opponents - even stopping short of ruling out a referendum.
The comments came ahead of the publication on Monday of a crucial report on the issue by an all-party group of peers and MPs.
The Joint Committee is expected to call for an 80% elected chamber, where members serve non-renewable 15-year terms. They would get a salary of around £50,000, rather than the existing attendance allowances.
It is also set to say that the constitutional significance of the shake-up means a referendum should be held.
Mr Clegg, who is championing the changes, is understood to be ready to give some ground, including agreeing to have 450 members in the new House, rather than the 300 he initially favoured. Some 90 of those could be appointed.
The Lib Dem leader could also support allowing peers to hold second jobs outside Parliament.
However, such concessions are unlikely to win over many Tories and Labour sceptics.
Reports suggest that at least six Cabinet ministers - Philip Hammond, Iain Duncan Smith, Michael Gove, Eric Pickles, Owen Paterson and Lord Strathclyde - would prefer to see the changes delayed. The party's backbenchers are also threatening a mass revolt on the issue, with the potential for resignations by several ministerial aides.