Speaker Mike Johnson has dropped one of the most aggressive options the G.O.P. was considering to cut Medicaid costs to help pay for President Trump’s domestic agenda, bowing to pressure from politically vulnerable Republicans and underscoring the deep party divisions imperiling the plan.

Leaving his office on Tuesday night after meeting with a group of more moderate members, Mr. Johnson told reporters that House Republicans had ruled out lowering the amount the federal government pays states to care for working-age adults who became eligible for the program through the Affordable Care Act’s Medicaid expansion.

Mr. Johnson also suggested he was leaning against another way of reducing spending on Medicaid, by changing the way the federal government pays states — currently by providing a percentage of beneficiaries’ medical bills — to a flat fee per person.

“I think we’re ruling that out as well, but stay tuned,” the speaker said.

The retreat was an acknowledgment that many House Republicans viewed the ideas — both of which would create large state budget shortfalls — as politically toxic. It also underscored how difficult it will be for Mr. Johnson’s conference to find Medicaid cuts that hit the spending targets Republicans set for themselves and also win enough votes to pass.

Ultraconservative Republicans quickly vented their opposition, in a public reminder that Mr. Johnson’s efforts to stave off a revolt of mainstream lawmakers could cost him crucial support from his right flank. That could doom Mr. Trump’s vast tax and spending cut plan in the House, where the speaker can afford to lose fewer than a handful of votes.

“Well - I haven’t ruled it out,” Representative Chip Roy, Republican of Texas, declared on social media after Mr. Johnson’s comments on Tuesday night about abandoning the idea of cutting back on federal payments for some Medicaid beneficiaries. “It’s necessary to stop robbing from the vulnerable to fund the able-bodied.”