Washington — Republican efforts to overhaul the nation’s health care system collided on Tuesday with fierce resistance about how it would affect people with pre-existing medical conditions, casting the proposal’s future into deeper uncertainty as GOP 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 pre-existing conditions, as is possible under the latest GOP 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 GOP leaders earlier this year.

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

In an interview with WHTC radio in Holland, Mich., Upton, a former chairman and current member of the House Energy and Commerce Committee, said he opposes the House GOP 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 GOP 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 pre-existing 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 GOP 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 pre-existing 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.