“I just want to let the world know I am 100 percent in favor” of the measure, Mr. Trump said at the White House after meeting around a dozen House lawmakers and shaking hands on revisions.

President Donald Trump agreed to add fresh Medicaid curbs to the House Republican health care bill on Friday, bolstering the measure with support from some conservative lawmakers but leaving its prospects wobbly. House leaders discussed other amendments calibrated to round up votes and scheduled a showdown vote Thursday.

“I just want to let the world know I am 100 percent in favor” of the measure, Mr. Trump said at the White House after meeting around a dozen House lawmakers and shaking hands on revisions. “We’re going to have a health care plan that’s going to be second to none.”

While the rapid-fire events seemed to build momentum for the pivotal GOP legislation, its fate remained clouded. One leading House conservative said the alterations were insufficient and claimed enough allies to sink the measure, and support among moderates remained uncertain.

“My whip count indicates that there are 40 no’s,” enough to defeat the bill, said Rep. Mark Meadows, R—N.C., who leads the hard—line House Freedom Caucus. He said the change “doesn’t move the ball more than a couple yards on a very long playing field.”

Across the Capitol, Sen. Dean Heller, R—Nev., facing re-election next year, became the fourth Republican senator to announce his opposition. That left Senate GOP leaders at least two votes shy of what they’d need to prevail in the chamber, which they control 52—48.

Congressional Democrats remain solidly opposed to the GOP effort.

Thursday will mark the seventh anniversary of when Obama signed his health overhaul into law, one of his milestone achievements enacted over unanimous GOP opposition. Beyond symbolism, Republican leaders want Congress to complete the measure before an early April recess exposes lawmakers to two weeks of lobbying and town hall pressure tactics by activists, doctors, hospitals and other opponents.

The Republican bill would kill much of former President Barack Obama’s health care law, including tax penalties for people who don’t buy insurance and its expansion of Medicaid, the federal-state health program for the poor. It would create new tax credits that would be less generous than current federal subsidies for many consumers, and repeal levies on the wealthy and medical firms that helped finance Obama’s expansion of coverage to 20 million Americans.

Trump’s deal with lawmakers would let states impose work requirements on some of Medicaid’s roughly 60 million recipients. The condition would apply to healthy people with no dependents, a White House official said.

The agreement would let states accept lump-sum federal payments for Medicaid, instead of money that would grow with the number of beneficiaries. The program currently costs the federal government around $370 billion annually and covers costs no matter the amounts.

Also, any additional states that expand Medicaid would not receive the additional federal money Obama’s law provided them for doing so. Thirty-one states have enlarged their Medicaid rolls under the law.

“These changes definitely strengthen our numbers,” said the House GOP’s top vote counter, Rep. Steve Scalise of Louisiana. “But they also show that President Trump is all-in now” to help win converts.

Those accepting the agreement included Rep. Mark Walker, R—N.C., leader of the Republican Study Committee, a large group of House conservatives.

It seemed clear that GOP leaders remained short of the 216 votes they’ll need, and additional changes were in the works.

Rep. Tom McClintock, R—Calif., said House Speaker Paul Ryan, R—Wis., assured him that the bill’s tax credit would be focused more on lower—income people. Rep. Robert Aderholt, R—Ala., among those who met with Trump, said the president “told his people” to work on changes making the measure more generous for lower-earning and older Americans.

Conservatives seemed unlikely to achieve their demands that the GOP bill’s phase-out of Obama’s Medicaid expansion now 2020 be accelerated to next year and that the credit be denied people with little or no tax liability. Centrists remained wary of yanking constituents from coverage. Many represent states where voters have gained Medicaid and other insurance under the 2010 statute.

Freshman GOP Rep. John Katko from a closely divided district in New York’s Hudson River valley said late Friday that he would oppose the measure, saying it would provide inadequate insurance access, hurt hospitals and not control costs.

In a report that weakened GOP support, the nonpartisan Congressional Budget Office has said the legislation would leave 24 million people uninsured in a decade and boost out-of-pocket costs for many.