The House is set to vote on Thursday on whether to limit abortion access, bar transgender services and end diversity training for military personnel, part of a series of major changes that hard-right Republicans are seeking to the annual defense policy bill, including yanking U.S. aid to Ukraine.

The debate was unfolding after Speaker Kevin McCarthy capitulated late Wednesday to a small group of ultraconservative Republicans who had threatened to block the legislation, which provides a yearly pay raise for U.S. troops and sets Pentagon policy, if their proposals did not receive consideration.

Instead the House moved forward on Thursday, with the fate of the $886 billion bill still in doubt. The far right’s proposals to strip away military assistance for Ukraine stand little chance of passage given the strong bipartisan consensus behind the aid, but it was not clear whether a proposal to bar the Biden administration from sending cluster munitions might draw enough bipartisan backing to succeed.

And the measures imposing socially conservative policies on the Pentagon are extremely popular among Republicans. Should they pass, Democrats are likely to abandon the bill in droves, sinking it altogether.

Shortly before debate got underway, Representative Katherine M. Clark of Massachusetts, the Democratic whip, told CNN that there would be no support for the bill in her party if it contained the provision to bar the Pentagon from providing time off and reimbursement to service members traveling out of state to get an abortion or other reproductive health services.

It was an unusual situation for the defense bill, normally a bipartisan matter that is considered one of the few must-pass items to come before Congress. This year, with Republicans in control of the House, it has become a partisan battleground whose very survival is in doubt.

“It is outrageous that a tiny minority of MAGA extremists is dictating how we’re going to proceed,” Representative Jim McGovern, Democrat of Massachusetts and the ranking member of the Rules Committee, said early Thursday morning, denouncing G.O.P. leaders for accepting the demands of what he called “a dozen far-right wing nuts.”

“When you have a razor-thin majority in one half or one branch of government, you don’t get to dictate every single amendment that comes to the floor,” Mr. McGovern said. “Democracy means compromise.”

Republican leaders, who can afford to lose no more than four votes on their side if Democrats remain united, have been counting on Democratic votes to help pass the defense bill. Some of them have expressed frustration with hard-right lawmakers’ demands to load the bill with a deeply conservative cultural agenda that could cost them those critical votes.

“We’ve got some people that want all the things that will cost us Democratic support, but won’t guarantee you yet, if they don’t get X, Y, or Z, that they will actually vote for final passage or even for a rule,” Representative Tom Cole, Republican of Oklahoma and chairman of the rules panel, said in an interview Wednesday.

Still, Mr. Cole said he would likely vote in favor of the socially conservative amendments.

The proposals have alienated some mainstream Republicans, including those from politically competitive districts. Their opposition could block the changes, potentially salvaging the defense measure.

And right-wing Republicans were expected to fail in their efforts to curtail military support for Ukraine. That included one proposal to end a $300 million military assistance program to train and equip Ukrainian soldiers that has been included in defense bills annually for almost a decade, and another to prohibit the United States from sending any other security assistance to Ukraine.

It was not clear whether a proposal to bar the Biden administration from sending cluster munitions to Ukraine, as the president announced he intended to do last week, could win the support necessary to pass.

Republican leaders have been agitating for cluster munitions to be sent to Ukraine for months, while most Democrats were outraged at President Biden’s decision. They argued that the unwieldy warheads — which scatter upon impact and routinely leave unexploded ordnance in the ground, endangering civilians for decades to come — would cost the United States the moral high ground in the war.

This week, a number of conservative Republicans aligned themselves with the Democrats opposing the move.