Tories are threatening a Commons revolt over George Osborne's plans to cut tax credits for low-paid workers after more of the party's MPs put pressure on the Chancellor to row back.
London mayoral candidate Zac Goldsmith and backbencher David Davis are among backers of a cross-party demand for extra protections who joined forces with Labour ex-welfare minister Frank Field to secure a debate later this month.
And the vote - due on October 29 - could take on extra significance if the House of Lords approves a move to refuse to back the reforms without further consideration by MPs.
Crossbench peer Baroness Meacher has dropped a plan to table a "kill" motion that would throw out the changes altogether, after coming under pressure to avoid what critics said would be a constitutional crisis.
But, with opinion on the issue running high among peers, she is expected instead to propose that a final decision by the upper chamber be delayed until after MPs have had another chance to debate it.
An Opposition-led Commons motion was easily defeated tonight - and attracted no Tory rebels - despite a string of David Cameron's party members, including several newly-elected MPs, criticising the policy and calling for a rethink.
Tory Heidi Allen used her maiden Commons speech to warn the changes go "too hard and too fast" and Johnny Mercer, a fellow member of the 2015 intake, urged the Chancellor to do "something, anything" to ease the "harshest" effects of the cuts on vulnerable people.
Former party chairman Baroness Warsi hailed Ms Allen's "brave and principled" speech, adding that tax credits "provided my parents the necessary buffer to allow us to study and work our way out of poverty".
Labour's opposition day motion, calling for a reversal of the policy, was defeated by Government MPs in the lobbies by 317 to 295, majority 22.
But the second Commons debate will present an opportunity for increasingly disgruntled Conservatives to make their feelings plain by seeking to overturn the Government's fragile majority on the issue.
In a joint statement, Mr Field and Mr Davis welcomed the decision of the backbench business committee "to grant us a debate in which we can vote on a House motion, rather than a party one.
"All the MPs requesting the debate wish to have a motion which is designed to help the Government meet its fiscal goals while supporting some of our most vulnerable constituents," they said.
The motion, which calls on the Government "to bring forward proposals that mitigate at nil cost the impact of its proposed changes to tax credits ... so as to protect the lowest paid workers", will be debated on October 29.
It is also signed by figures including Liberal Democrat leader Tim Farron, the Westminster leaders of the DUP, Plaid Cymru and the SDLP, the Scottish National Party's health spokeswoman Philippa Whitford and another two Tories, Stephen McPartland and Jason McCartney.
Shadow chancellor John McDonnell said the issue could be as damaging to the Tories as the Liberal Democrats ditching a pledge not to increase student tuition fees in joining the coalition government in 2010.
"David Cameron needs to listen to MPs in his own party on the tax credit cuts.
"Heidi Allen is following other Tory MPs like David Davis, Boris Johnson and many more who are waking up to what Labour has been saying: it's simply not fair to make families £1,300 a year worse off to pay for tax breaks for the few.
"The Tories risk following the Lib Dems in breaking pre-election promises to the British people, and voters won't forget the broken promises made during this year's election campaign any more than those made by the Lib Dems on tuition fees in 2010."