The focus of that scramble is Representative Fred Upton of Michigan, the former chairman of one of the House committees that drafted the American Health Care Act, who has a long history of negotiating with Democrats on large health care measures, like the new 21st Century Cures Act. Mr. Upton said on Tuesday that the latest version of the health care bill “torpedoes” protections for people with pre-existing medical conditions and had asked for at least $8 billion to supplement funds already in the bill to help state governments struggling with hard-to-insure populations.
The extra spending could anger the most conservative members of the House, who had recently come around to supporting the bill. Last-minute spending increases and special provisions in 2010 to win over Senate Democrats to the Affordable Care Act had stoked outrage among conservatives who fumed at “the Cornhusker kickback” and the “Louisiana Purchase.”
But Mr. Upton, who led the House Energy and Commerce Committee as the repeal movement built steam, is crucial. He had declared on a local radio show, “I cannot support this bill with this provision in it,” just as Mr. Ryan was insisting that the legislation would protect the sick.
Winning back Mr. Upton, who has served in the House for 30 years, would buoy Republican leaders, who hope to get the bill through the House by Thursday, before lawmakers go home again and face pressure from constituents. Party leaders are facing an onslaught of advocacy groups and Democratic attack ads saying the bill would harm the nation’s most vulnerable citizens. Even a late-night talk show host, Jimmy Kimmel, joined with an emotional appeal.
A tearful Mr. Kimmel on Monday night told the story of his infant son, Billy, who was born with heart defects and had surgery. Mr. Kimmel pleaded with Congress not to undermine the Affordable Care Act’s ban on discrimination against people with pre-existing conditions.
After Mr. Kimmel’s monologue went viral, former President Barack Obama weighed in on Twitter, writing: “Well said, Jimmy. That’s exactly why we fought so hard for the ACA, and why we need to protect it for kids like Billy.”
House Republican leaders are also fighting against the clock. The House is scheduled to be in recess beginning on Friday and is not set to return until May 16. Republicans who are on the fence are likely to get an earful from their constituents.
“I think it’s imperative that we have a vote before we leave for a week,” said Representative Mark Meadows, Republican of North Carolina and the chairman of the conservative House Freedom Caucus.
In the radio interview, Mr. Upton was explicit: Concessions made to win over the hard-line members of the Freedom Caucus were costing the leadership support from more moderate Republicans. He said “there are a good number of us that have raised real red flags and concerns.”
Mr. Trump, whose advisers have been pressing aggressively for a vote on the health care overhaul, seemed oblivious of the latest setback for the measure on Tuesday.
“How’s health care coming, folks, how’s it doing — all right?” Mr. Trump said, addressing Republican lawmakers attending a trophy award ceremony in the White House Rose Garden for the United States Air Force Academy’s football team. “We’re moving along? I think it’s time now, right?”
After visiting the Capitol on Monday, Vice President Mike Pence returned on Tuesday, trying to corral votes for the repeal bill. Mr. Ryan insisted that Republican leaders were “making very good progress with our members,” but he offered no indication of when a vote might be held.
Republicans were clearly divided over the adequacy of the bill’s protections for people who are sick or disabled.
“There are a few layers of protections for pre-existing conditions in this bill,” Mr. Ryan said.
At the heart of the debate is an amendment to the repeal bill proposed by Representative Tom MacArthur, Republican of New Jersey. The amendment, which won over the Freedom Caucus last week, would give state governments the ability to apply for waivers from the existing law’s required “essential health benefits,” such as maternity, mental health and emergency care, and from rules that generally mandate the same insurance rates for people of the same age, regardless of their medical conditions.
With a waiver, states could permit insurers to charge higher premiums based on the “health status” of a person who had experienced a gap in coverage. To qualify for a waiver, a state would have to have an alternative mechanism, like a high-risk pool or a reinsurance program, to provide or subsidize coverage for people with serious illnesses.
“States can’t leave people with pre-existing conditions high and dry,” Mr. MacArthur said Tuesday, defending his proposal.
But the MacArthur amendment has distressed some Republicans because of concerns that it would allow states to gut protections for consumers.
Representative Tom Rooney, Republican of Florida, said he was “leaning yes” on the repeal bill, but agonizing over how to explain his vote to constituents.
“I have a lot of people who call my office on a daily basis who are extremely angry,” he said. “It’s not just because I’m a Republican, but because they are sincerely scared.”
Many people with pre-existing conditions fear that they may lose coverage and “are going to die because of a vote we might be taking,” Mr. Rooney said.
The Freedom Caucus had pushed hard to roll back federal insurance requirements.
“The pre-existing condition debate and discussion in Congress, far as I’m concerned, is over,” Representative Scott Perry, Republican of Pennsylvania and a member of the Freedom Caucus, said Tuesday. “They are covered; we acknowledge it; we provide for it; it is done.”
The White House threw a hand grenade into the delicate negotiations over health care on Tuesday when Mr. Trump’s budget director, Mick Mulvaney, suggested the administration might take action that would undermine the Affordable Care Act, with or without Congress.
Mr. Mulvaney raised doubts about whether the federal government would continue making certain payments to insurers. The payments enable insurers to reduce deductibles and other out-of-pocket costs for low-income people, a form of assistance known as cost-sharing reductions.
Discussing a bipartisan agreement in Congress to fund the federal government for the next five months, Mr. Mulvaney said, “There’s absolutely no language in this bill that requires us to make any Obamacare bailout payments, any C.S.R. payments of any way, shape or form as a result of this deal, O.K.?”
Asked whether the Trump administration would stop making the payments, he said, “We’ve not made any decisions at all on May.”
The White House Office of Management and Budget later said Mr. Mulvaney meant to say that the administration had made no commitment to pay the subsidies beyond May.
The House Democratic whip, Steny H. Hoyer of Maryland, said Mr. Mulvaney’s comments undermined confidence in insurance marketplaces and criticized the Trump administration. “Its actions, continuing to sabotage the Affordable Care Act, will inevitably force premiums to skyrocket, hurting consumers,” Mr. Hoyer said.
Congress’s inability to agree on health care legislation is already sending tremors through insurance markets, making it much more difficult for insurers to plan for 2018.
Monday was the deadline for insurers in California to file preliminary information on rates and benefits for next year. Dave Jones, the California insurance commissioner, said he had taken “the unprecedented step of authorizing health insurers to file more than one set of proposed rates for 2018 — one assuming the A.C.A. is enforced and funded, and the other assuming that President Trump and House Republican leaders continue to undermine or repeal the law and cause unnecessary premium increases.”
Even as some Republicans have come out in opposition to the repeal bill in recent days, the Trump administration and House Republican leaders have also picked up support from other party members.
Representative Paul Gosar, Republican of Arizona, said Tuesday that he had switched to yes after receiving assurances that the Senate would vote on one of his bills, which would scale back the federal antitrust exemption for health insurance companies.