Mr. Moran, who was chairman of the Senate Republicans’ campaign arm when they won control of the chamber in 2014, is considered a durable party loyalist.

“If you lose Jerry Moran, you’ve lost part of the mainstream of the party,” said Burdett Loomis, a political-science professor at the University of Kansas. “If guys like him start unraveling, not just on health care but on other issues, McConnell has no chance of controlling that caucus.”

But Mr. Moran, in opposing the bill, risks alienating conservatives — though a primary challenge in 2022 is hard to imagine.

“I think for Senator Moran and others like him, it’s not enough to say, ‘Well, I don’t like the current bill,’” said former Representative Tim Huelskamp, a Republican who succeeded Mr. Moran in representing Kansas’ vast First District. “Tell me how you’re going to get to the point where the bulk of Obamacare is repealed, and when will you get it done?”

“We’ve waited six months plus seven years,” he added, “and now it’s time to get some action done.”

Kathleen Sebelius, a former Democratic governor of Kansas who was secretary of health and human services under President Barack Obama, expressed skepticism that Mr. Moran would remain opposed to the bill.

“I’m afraid at the end of the day, he will vote for some kind of legislation,” she said, adding of the bill’s being revised, “A really awful bill, if it gets a little less awful, is still awful.”

Kansas’ other Republican senator, Pat Roberts, offered an assessment of the Senate bill strikingly different from Mr. Moran’s: Kansas, he said, “fared well under this draft.” Before the recess, according to The Associated Press, he offered an evocative description of how Republican senators were trying to come to agreement.