Ministers are today braced for a potentially damaging Commons defeat over Nick Clegg's plans for House of Lords reform.
The Deputy Prime Minister received a mauling from Tory backbenchers on Monday as he opened a two-day debate which could have far-reaching consequences for the coalition.
Afterwards aides admitted their chances of winning the crucial vote tonight on a timetable motion - limiting the time for debate in the Commons - were no better than 50-50.
"It is close - unquestionably," said one aide to Mr Clegg.
Rebel Tories are threatening to combine with Labour to vote against the timetable motion - seen as essential if the House of Lords Reform Bill is not to be "talked out" by opponents making marathon speeches.
Although the Bill has the backing of both the Conservative and Liberal Democrat leaderships, it is regarded as vital to the Lib Dems after their defeat in the referendum on AV voting reform for parliamentary elections.
Downing Street has confirmed all members of the Government - including unpaid ministerial aides - would be expected to vote for the legislation.
However one opponent - Conor Burns, the parliamentary private secretary to Northern Ireland Secretary Owen Patterson - has already written to the Tory Chief Whip Patrick McLoughlin putting his job "at his and the PM's disposal".
Around 70 Conservatives backbenchers had signed a letter warning the plans for a mainly elected upper chamber threatened a "constitutional crisis" and calling for the Bill to be given "full and unrestricted" scrutiny.
The letter sent to other Tory MPs on Monday morning by the rebels reads: "We come from all sides of the Conservative Party, an are writing as reformers to express our serious concern and the current proposals to create an elected House of Lords.
"What is now proposed will undermine the primacy of the Commons, with competing chambers which will lead to legislative gridlock.
"It will create hundreds of unaccountable new elected politicians at a time when we as a party are committed to reducing the cost of politics.
"And it will produce a chamber which is less expert, less diverse and significantly more expensive than the present one."
Jonathan Ashworth MP
Liz Kendall MP
Signatories included a number of select committee chairmen - among them Bernard Jenkin, John Whittingdale, James Arbuthnot and Bill Cash - as well as the former shadow home secretary David Davis.
Opponents of reform argue that an elected House of Lords could undermine the traditional primacy of the Commons, leading to constitutional deadlock.
Opening the second reading debate on the floor of the Commons chamber yesterday, Mr Clegg insisted he was not creating "some kind of monster" and was simply trying to reform a "flawed institution".
However he struggled to make progress in the face of a barrage of hostile interventions from Tory MPs massed on the benches behind him.
Afterwards, the Lib Dem Foreign Office Minister Jeremy Browne indicated that while the coalition would survive the vote, there would be "consequences" if Tory MPs succeeding in defeating the timetable motion.
"We are not making threats," he said.
"I think the coalition is in pretty safe condition. I don't think we should exaggerate what it happening over the next couple of days, but it is with consequences.
"The Liberal Democrats have been very honourable, very disciplined, in terms of supporting the Government as a whole and I think Conservative MPs need to be similarly honourable and disciplined."