Coalition ministers are struggling to secure a much-needed victory on Lords reform, as Nick Clegg's legislation approaches its first major test in the Commons.
MPs will vote on Tuesday night after the first general debate on the bill. The critical division comes over the programme motion, a standard procedural device which sets out a limited period of debate for the legislation.
With Labour having opted to oppose the programme motion on the grounds that the constitutional reform requires lengthy scrutiny in parliament, up to 100 Tory rebels could see the government defeated outright.
A letter from nine former Conservative ministers, including Norman Lamont and Geoffrey Howe, has encouraged Tory MPs to vote against Clegg's bill.
Intensive work by government whips has reportedly reduced the number of rebels to around 80 but a defeat for ministers remains a real possibility, however.
Liberal Democrats have responded by adding pressure of their own to coalition colleagues to secure the vote.
Friday saw Clegg's departing adviser Richard Reeves threaten the withdrawal of support for boundary changes if the Lib Dems do not receive support on Lords reform.
That has prompted claims of Lib Dem 'blackmail' from Tories, who viewed the boundary changes as being a quid pro quo for the referendum on electoral reform delivered – and defeated – in May 2011.
Now the coalition's junior party are making the issue a personal one, with the prime minister targeted this morning.
"A defeat for the government would be the first since the coalition was formed – and if it happened this would be down to Tory MPs," a source close to Clegg told the Sunday Telegraph newspaper.
"Nick [Clegg] has already delivered Lib Dem support on difficult issues such as the NHS and welfare reform – it's now time for David Cameron to do the same. This is ultimately a test of his leadership."
If the programme motion is defeated a bitter struggle over parliamentary time will commence, with the coalition's legislative agenda at stake.
Business secretary Vince Cable did his best to play down prospects of a rift this morning.
He told BBC1's The Andrew Marr Show: "We are not talking about walking away. That isn't an issue.
"But there is absolutely no reason why this vote should be lost. All three parties agree that we should have an elected House of Lords, that the present system is completely unsustainable.
"There is no point engaging in a hypothetical debate about something I don't think will actually happen."