The Liberal Democrats have threatened to block boundary changes if plans for an elected House of Lords are killed off by the Conservatives.
Richard Reeves, a senior aide to Nick Clegg, warned there would be "broader consequences" for the coalition if it fails to win a crucial vote on Lords reform next week.
Up to 100 Conservative backbenchers could object to the Deputy Prime Minister's plans for a mainly-elected upper chamber, in what would be the biggest revolt in the party's history.
Tories deeply against the proposals are threatening to combine with Labour to defeat a programme motion designed to cut short debate on the legislation in the Commons.
The motion, which will be put to the vote on Tuesday, outlines a timetable which is aimed at making sure the Bill proceeds through Parliament.
The Lib Dems believe there will only be a tiny chance of the reforms becoming law if it is defeated because opponents will then be free to make lengthy speeches in a bid to kill the measures.
Mr Reeves, in an interview with The Independent as he stepped down as Mr Clegg's director of strategy, said that without the time limit the chances of the Bill reaching the Statute Book are "vanishingly small".
He made clear, if that happened, the Lib Dems would block plans to redraw parliamentary boundaries and slash MP levels from 650 to 600 - a move which could cost the Tories an extra 20 seats at the next general election.
"It is a very serious moment for the Government," Mr Reeves said. "The vote is hugely significant. It is the critical moment for Lords reform, a once-in-a-generation chance to secure it.
"There would be broader consequences for the Government's programme, particularly around political and parliamentary reform.
"The idea that failure to deliver a Government commitment on Lords reform would be consequence-free is for the birds."
He added: "Anyone who thinks Nick Clegg will shrug his shoulders, say 'never mind' and 'everyone tried our best', will be in for a rude awakening. That is not going to happen."
Mr Reeves, who is moving to the US, told the paper that Lords reform "goes to the heart of what coalition is about - making and keeping deals".
But, despite the tough talk, he said the Lib Dems would not walk out of the coalition, which would be "crazy", or block Tory public service reforms.
The draft Bill, which would create an 80% elected upper house and slash membership from 800 to 450, is the last remnant of Lib Dem constitutional reform plans.
If it became law, it would complete the removal of hereditary peers from the Lords. Elected members would be introduced in groups of 120 at the next three elections.
The process would be completed by 2025 and elected members would serve for a 15-year term.
Crucially for the Lib Dems, members would be elected by a form of proportional representation. This all but guarantees that no party would have a majority, meaning the Lib Dems would always have a key influence.
The Government has insisted its MPs will be whipped to support the Bill and that any ministerial aides who refuse to do so will have to resign or be sacked.
Prime Minister David Cameron said last week: "We have been discussing this issue for 100 years and it really is time to make progress.
"There are opponents of Lords reform in every party. But there is a majority in this House for a mainly-elected House of Lords and I believe there's a majority for that in the country.
"If those who support Lords reform don't get out there and back it, it won't happen. That is the crucial point."