WASHINGTON — Republican efforts to overhaul the nation’s health-care system collided Tuesday with fierce resistance about how it would affect people with pre-existing medical conditions, casting the proposal’s future into deeper uncertainty as Republican leaders scrambled to try to salvage it.

On Capitol Hill, influential Rep. Fred Upton, R-Mich., came out against the plan, dealing a major blow to proponents trying to secure enough votes to pass it in the House. Across the country, late-night host Jimmy Kimmel’s emotional story about his newborn son’s heart condition reverberated on television and the Internet. And former President Barack Obama, who signed the bill Republicans are trying to dismantle, took to Twitter to defend it.

All three voiced concerns about losing a core protection in the Affordable Care Act for people with preexisting conditions, as is possible under the latest Republican plan. Such growing worries threatened to derail the revamped attempt to revise key parts of the ACA – or at least send Republicans back to the drawing board.

“I do think each minute that has passed, each hour and each day, the ‘no’ members are becoming more locked in ‘no,’ and we may be losing members,” said Rep. Chris Collins, R-N.Y., who favors going back to the original version of the American Health Care Act that was scrapped by Republican leaders earlier this year.

Republicans left their weekly conference meeting Tuesday with no health-care vote on the schedule. The House is slated to recess Thursday until May 16.

In an interview with WHTC radio in Holland, Michigan, Upton, a former chairman and current member of the House Energy and Commerce Committee, said he opposes the House Republican plan because it “torpedoes” safeguards for people with pre-existing conditions.

“I told the leadership I cannot support the bill with this provision in it,” Upton said. “I don’t know how it all will play out, but I know there are a good number of us that have raised real red flags.”

A Washington Post analysis shows 21 House Republicans either opposed to or leaning against the bill, and 22 more either undecided or unclear in their positions. If no Democrats support the bill, the Republicans can lose no more than 22 Republican votes to pass it in the House.

Upton’s comments came a day after Rep. Billy Long, R-Mo., a longtime opponent of the ACA, voiced similar concerns as he came out against the latest plan. On Tuesday, Long said the preexisting-condition provision was the sole reason for his opposition.

“They take that out, put the vote on the floor that they pulled, and I’m with them,” Long said, referring to the first version of the bill, which House Republican leaders withdrew in March after it was clear that it lacked the support to pass the chamber.

After the failure, Republicans renegotiated and opted to add an amendment to the bill that would enable insurers to deny coverage or charge more to people with preexisting conditions if their states opted out of provisions in the ACA barring such decisions. The states would have to set up “high-risk pools” to absorb some of the costs of caring for those people.

The idea was to find a middle ground that would attract conservative Republicans who want to do away with as many ACA regulations as possible and centrist Republicans who worry about stripping vulnerable populations of the coverage they receive under the ACA.

That balance has been very hard to reach. And external pressure has made it no easier.

On his show Monday night, an emotional Kimmel repeatedly teared up as he told the story of his newborn son Billy’s surgery for a heart defect. The procedure was successful but shook him.

Kimmel encouraged lawmakers not to threaten the protections people with preexisting conditions receive under the ACA.

“Whatever your party, whatever you believe, whoever you support, we need to make sure that people who are supposed to represent us – and people who are meeting about this right now in Washington – understand that very clearly. Let’s stop with the nonsense. This isn’t football, there are no teams,” he said on ABC’s “Jimmy Kimmel Live.”

Kimmel tweeted a link to the video Monday night. By Tuesday evening, it had received more than 4.2 million views on YouTube. Among those who had seen it: the 44th president.

“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,” Obama wrote on Twitter.

“I read about it,” said Rep. Leonard Lance, R-N.J., of Kimmel’s viral speech, though he said he had not seen it. Lance opposes the current Republican plan.

President Donald Trump, who has shown an eagerness to swiftly pass a health-care bill, continued pressing congressional Republicans to act. On Tuesday, Vice President Mike Pence traveled to Capitol Hill again to coax legislators to support the bill. Trump called lawmakers from the White House.

“How’s health care coming, folks? How’s it doing? All right. We’re moving along? All right. I think it’s time now, right? Right?” he said after name-checking some lawmakers in attendance as he presented the U.S. Air Force Academy football team with the Commander-in-Chief’s Trophy at the White House.

Trump has also added confusion to the debate, saying in an interview that aired Sunday on CBS’s “Face the Nation” that the health-care plan would “beautifully” protect those with preexisting conditions.

House Speaker Paul Ryan, R-Wis., appeared keen to avoid signs of discord Tuesday, telling reporters that Trump has been “nothing but helpful” on health care. He and his top lieutenants also tried to defend the plan against criticism that it would harm Americans with preexisting conditions.

“Our bill protects people with preexisting conditions, and it actually provides multiple layers of protection for people with preexisting conditions in ways that Obamacare doesn’t do,” House Majority Whip Steve Scalise, R-La., told reporters.

Scalise’s defense was that current law offers such protections and any states opting out “actually have to lay out how they are going to protect people with pre-existing conditions.”

Rep. Paul Gosar, R-Ariz., a member of the far-right House Freedom Caucus who did not support the first GOP proposal but does back the new one, said he was hearing that a new amendment would add money for the high-risk pools – though he didn’t know how they would be paid for. Collins said he heard something similar, but he was pessimistic that the differences could be ironed out.

“I’ve heard it, but I don’t believe it’s a dollar-and-cents issue,” Collins said.

Some reluctant Republicans continued to talk throughout the day as if the bill could still be negotiated.

“Hopefully we’re just a handful away,” said Rep. Mark Meadows, R-N.C., the chairman of the Freedom Caucus, who helped put together the latest proposal. “There’s still a lot of undecided, but generally, at this stage of the game, you can address the concerns of the undecideds.”

Rep. Tom MacArthur, R-N.J., a co-chairman of the centrist Tuesday Group who negotiated the new proposal with Meadows, dodged questions about possible further changes. “I’m not in every conversation, so I can’t really say,” he said.

Rep. Elise Stefanik, R-N.Y., another Tuesday Group co-chairman, who has not taken a position on the bill, told reporters that she was “involved in all of the discussions.”

Appropriations Committee Chairman Rodney Frelinghuysen, R-N.J., who opposed the earlier version of the bill, evaded repeated questions about whether he would vote yes now.

“The position I’m taking is that the most important thing is to keep the government open for business,” he said, referring to a different bill to keep the government funded through September.

In the Senate, where the measure is likely to face an even steeper climb if it makes it there, Majority Leader Mitch McConnell, R-Ky., painted a less-than-rosy picture of its outlook.

“It’s no secret that this has been a big issue in the last four campaigns, and we’re going to continue to work on it,” McConnell said. “And when they send it over here, it’ll be a real big challenge on the Senate side as well.”

Back in the House, the difficulty of the more immediate hurdles was as clear to advocates of the proposal as to its detractors.

“The most sincere anger I’ve noticed comes from people who are sincerely scared, people who may have a pre-existing condition who feel like they’re about to lose (coverage) and they’e going to die, and they’re going to die because of a vote that we might be taking,” Rep. Thomas Rooney, R-Fla., who supports the current plan, told reporters.

“If we cannot explain to people that is not going to happen, then it is going to be very difficult to ever bring a bill to the floor.”