Plans put forward by Deputy Prime Minister Nick Clegg to reform the House of Lords will be put to the test in a crucial vote by MPs later.
On Monday, Mr Clegg set out government plans for a smaller and mostly elected second chamber of Parliament.
Many Tory and Labour backbenchers spoke out against the planned changes.
Up to 100 Tory backbenchers are thought to be prepared to vote against a timetable motion, limiting the time for debate on the bill in the Commons.
If Conservative rebels succeed, with Labour help, in throwing out the timetable in getting the legislation through parliament it could be enough to delay the plans for months or even years.
'Sham democratic chamber'The BBC's Sean Curran said ministers face the prospect of "their first significant defeat in the Commons" which would "force a crisis in the coalition".
The government's "programme motion" would limit the amount of time MPs can spend discussing the issue to 10 days.
A letter opposing the current plans, signed by 70 Conservative MPs, has called for "full and unrestricted scrutiny" of the proposed legislation which will "pile a constitutional crisis on top of the economic crisis".
Lords reform: At-a-glance
- A smaller chamber - reduced from 826 members to 450.
- The majority, 80%, of members would be elected - at the moment nearly all peers are appointed either by political parties or by the independent House of Lords Commission.
- But 90 members, 20%, would still be appointed, by an Appointments Commission, on a non-party basis.
- Time-limited membership - Once elected, peers would serve a non-renewable 15-year term instead of being members for life.
- A reduced number of bishops - The number of Church of England bishops would be cut from 26 to 12.
- No more Lords and Baronesses - The chamber would still be called the House of Lords but members would not have the title "Lord". Parliament to choose a new name for members.
Signatories to the letter include a number of MPs elected in 2010, plus former Foreign Secretary Malcolm Rifkind and select committee chairs Bernard Jenkin, James Arbuthnot and Bill Cash.
During Monday's debate, Mr Rifkind told MPs that the government's proposals would create a "a sham democratic chamber which will consist overwhelmingly of members who would rather be in this chamber".
"I believe this bill has to be opposed because essentially what it is seeking to do will damage the fabric of our government," he said.
Labour says it will oppose the programme motion while Conservative MPs could be forced to resign from any front bench jobs if they rebel.
'Foot dragging'On Monday, amid raucous exchanges in the Commons, Mr Clegg urged MPs to support democratic reform of the House of Lords, which he said was needed to "get a grip" on the rising numbers of peers and make the chamber legitimate.
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Deputy Prime Minister Nick Clegg: "This bill is about fixing a flawed institution"
He said: "There will be those who are not interested in rational discussion. Those who will oppose Lords reform in whatever form, at whatever time, in whatever century, no matter what commitments their parties have made.
"This project has always been dogged by those who fear change."
Mr Clegg rejected the criticism of the amount of time set aside to debate the bill, saying there should be no more "foot dragging" on a project that had already lasted 100 years.
Labour and the Liberal Democrats promised to reform the Lords in their 2010 general election manifestos, while the Conservatives pledged to "work to build a consensus" on reform.
Speaking to the BBC, Labour leader Ed Miliband defended his party's position amid Lib Dem accusations of opportunism for planning to oppose the timetabling motion.
"I have said we want proper scrutiny of these proposals and will ensure they get into the House of Lords to be debated. I am not saying this bill will die in the House of Commons. I don't want that to happen."
The Conservative rebels say their party's manifesto pledge - and the coalition agreement's promise to "bring forward proposals" for an elected House of Lords - have already been fulfilled so they are not breaking any commitments by voting against the bill.
