It took everything House Republicans could muster to just barely nudge their major domestic policy bill over the finish line by the sparest of margins at daybreak on Thursday.

Fresh off the triumph, they face a distinctly less appealing prospect: that the fragile deal they struck to pass it, which could carry substantial political risks for some of their most vulnerable members in next year’s midterm elections, is about to be dismantled by the Senate and replaced with something even less palatable.

Long before Speaker Mike Johnson’s herculean effort paid off with the House’s 215-to-214 approval of the One Big Beautiful Bill Act, Democrats had promised to make the measure — with its cuts to popular programs including Medicaid and food assistance — a central line of attack against the G.O.P.

And within hours afterward, Senate Republicans made clear that the debate within their party over the measure was far from over and that there would be significant changes in any measure they managed to send back to the House. Some of those could easily alienate House Republicans who already swallowed considerable reservations about the bill to vote “yes.”

If Democrats were brutal in their assessment of the House-passed legislation, some of the Senate Republican critics were unsparing, as well. Senator Ron Johnson of Wisconsin called the deficits produced by the House bill “completely unacceptable.”