WASHINGTON — The Republican health-care overhaul faces its greatest test ever Thursday as President Trump and House Speaker Paul D. Ryan, R-Wis., work feverishly to persuade enough Republican lawmakers to back the measure and push it to a floor vote.

Late Wednesday, the White House and House leaders were still scrambling to boost support, and signaled at the 11th hour a willingness to rework the measure to mollify conservatives. On Thursday morning, House leaders postponed a 9 a.m. meeting of the entire GOP Conference, signaling that negotiations were still underway.

As of late Thursday morning, 36 House Republicans – mainly conservatives – had announced their opposition to the bill, known as the American Health Care Act.

After insisting for weeks that the changes sought by hard-right members would render the bill unable to pass the Senate, White House officials and GOP House leaders appeared to shift their thinking – and opponents agreed to keep working on a deal with the goal of holding a floor vote in the House by Thursday night.

Rep. Mark Meadows, R-N.C., chairman of the conservative House Freedom Caucus, said he had taken personal calls Wednesday from Trump seeking a resolution, although he said no formal offer had been extended by the White House.

“We are working very diligently tonight to try and get there,” Meadows said Wednesday.

“The president has been profoundly engaged,” said Rep. Trent Franks, R-Ariz. “I think things are going in a very good direction right now.”

Trump is scheduled to meet with Freedom Caucus members at 11:30 a.m. at the White House, and will then host a “listening session” on health care with truckers there Thursday afternoon.

Thursday marks the seventh anniversary of the Affordable Care Act, and Republican leaders are eager to mark it with a historic vote demonstrating its evisceration has begun. In a sign of how high the stakes are for both parties, former President Barack Obama issued a statement noting that more than 20 million Americans have gained coverage since he signed the law, while the rise in health costs has slowed.

“So the reality is clear: America is stronger because of the Affordable Care Act,” Obama said, adding that Republicans are welcome to work with Democrats to improve the law. “But we should start from the baseline that any changes will make our health care system better, not worse for hard-working Americans. That should always be our priority.”

Both the president and vice president made personal appeals throughout the day Wednesday to secure the votes needed to pass the House. While most of their lobbying is focused on the roughly two dozen House conservatives skeptical of the measure, a handful of moderates have also decried the current proposal as harming the elderly and poor.

Vice President Mike Pence huddled with members of the Freedom Caucus in his Eisenhower Executive Office Building office early Wednesday, while Trump met with 18 House Republicans at the White House, but these efforts appeared to produce just one definitive aye vote from the conservative camp: Rep. Steve King, R-Iowa.

GOP leaders can afford only 22 defections, given that one Democrat is expected to be absent Thursday. A Freedom Caucus spokeswoman said that “more than 25” members of the group oppose the bill.

Wednesday’s events laid bare party leaders’ struggle to muster enough votes for one of their defining goals: to roll back the 2010 health-care law that helped galvanize conservatives in the years since to wrest control of both the legislative and executive branches from Democrats.

If Republicans fail this initial test of their ability to govern, Trump and Capitol Hill Republicans may face a harder time advancing high-priority initiatives on infrastructure, tax reform and immigration. They might also find themselves navigating strained relationships among themselves.

For much of Wednesday, the Freedom Caucus’s message, spokeswoman Alyssa Farah tweeted, was: “Start over.”

At the same time, four more Republican moderates – Reps. Charlie Dent (Pa.), Frank A. LoBiondo (N.J.), Daniel Donovan (N.Y.) and David Young (Iowa) – announced their opposition Wednesday, increasing pressure on leaders to win over the conservatives.

Ryan summoned more than a dozen members of the moderate Tuesday Group to his office late Wednesday in an apparent bid to curb further defections. One participant, who spoke on the condition of anonymity to describe the private meeting, said Ryan and other House leaders described the potential deal with the Freedom Caucus, which would strip essential health benefits but leave other ACA mandates, such as those dealing with preexisting conditions and coverage of adult dependents, in place.

“People got to say their piece and react to the proposal. It’s safe to say people had concerns about stripping out essential health benefits, especially at this late hour,” the Tuesday Group member said. “I think they’re short [of votes], and I think they’re considerably short . . . I’m not sure where all this goes tomorrow.”

Conservatives are seeking to eliminate more of the ACA’s insurance mandates, known as “essential benefits,” which require plans to cover specific medical benefits, such as mental health care, prescription drugs and preventive care. That, conservatives argue, is the only reliable way to force down premiums.

Ryan warned in an interview Wednesday with conservative radio host Hugh Hewitt that fulfilling those GOP demands would violate Senate budget rules and leave the bill vulnerable to a blockade by Democrats.

“Our whole thing is we don’t want to load up our bill in such a way that it doesn’t even get considered in the Senate,” the speaker said. “Then we’ve lost our one chance with this one tool we have.”

That stance appeared to shift late Wednesday, when separate aides in the White House and the House GOP leadership said a new interpretation of Senate rules had raised the possibility that acceding to the Freedom Caucus’s request might not threaten Senate consideration of the whole bill. But both aides said the provision could still be stripped out once the bill reaches the Senate.

Democratic Senate aides insisted that would be the case. “What the proponents aren’t telling conservative House Republicans is that the plan to repeal essential health benefits will almost certainly not be permissible under Senate reconciliation rules,” said Matt House, a spokesman for Senate Minority Leader Charles E. Schumer, D-N.Y.

In fact, the new negotiations late Wednesday raised the possibility that the challenge would only grow at the other end of the Capitol. There are at least a dozen skeptics of the bill among Senate Republicans, who maintain a slim 52-to-48 advantage in the Senate, and many of them want to maintain some of the current law’s more generous spending components. Republicans can afford to lose the votes of only two senators, assuming Pence would step in to cast a vote for the health-care rewrite in the case of a tie.

In addition to conservatives, who do not think the proposal does nearly enough to undo the ACA, some moderates fear it will harm their constituents as well as their party’s prospects at the ballot box.

“We’re not there yet,” Meadows said Wednesday, “but we’re very optimistic that if we work around the clock between now and noon tomorrow that we’re going to be able to find some common ground. Tonight is an encouraging night, and yet I don’t want to be so optimistic to say that the deal is done, but I do think that there is a framework to work with our leadership and the leadership in the Senate and certainly the administration to find some common ground.”

He continued: “The overall impression of the Freedom Caucus is we’re willing to jump through unbelievable hurdles to hopefully get to a point where this bill is better for the American people.”

An additional potential hurdle facing the bill is the updated analysis still to come from the Congressional Budget Office, which will reflect changes to the measure that were issued Monday. That analysis could be rendered inaccurate if further changes are made before the vote.

Earlier Wednesday, even as opposition appeared to persist, White House press secretary Sean Spicer told reporters that the measure would pass the House, adding that there is no Plan B if the proposal goes down.

“There is Plan A and Plan A,” said Spicer, who described Trump as “the closer” for the deal. “We’re going to get this done.”

Complications stemming from the bill’s last-minute tweaks appeared to add yet another political headache Wednesday, as veterans groups discovered that the latest draft might make them ineligible for a tax credit. A change made to ensure the measure would comply with Senate rules created a separate consequence – that individuals would qualify for the bill’s tax credits only if they “are not eligible” for other types of coverage, including those provided by Veterans Health Administration.

In an email, House Ways and Means Committee spokeswoman Lauren Aronson said the issue would be fixed in subsequent legislation. “This amendment makes no change to veterans’ health care. In working with the administration and the Veteran Affairs Committee, we will continue to ensure that America’s veterans have access to the best care available.”

Carlos Fuentes, legislative director for the Veterans for Foreign Wars, said veterans want the issue resolved before any bill becomes law. “It would be a huge impact on veterans if this were not corrected,” he said.

Abby Phillip, Lisa Rein and Kelsey Snell contributed to this report.