WASHINGTON — The House steered toward a climactic vote Friday on the Republican health care overhaul, plunging ahead despite uncertainty over whether they had the votes to prevail in what loomed as a monumental gamble for President Donald Trump and his GOP allies in Congress.

Debate began hours after White House officials told fractious GOP lawmakers at a Thursday night Capitol meeting that Trump was finished negotiating with GOP holdouts and would move on to the rest of his agenda, win or lose.

In a morning tweet, Trump targeted the House Freedom Caucus, whose hard-right members have been the core of opposition to the GOP legislation and have come under intense pressure from the White House and party leaders to fall into line. The bill would replace major parts of President Barack Obama’s health care law and would block federal payments for a year to Planned Parenthood.

“The irony is that the Freedom Caucus, which is very pro-life and against Planned Parenthood, allows P.P. to continue if they stop this plan!” Trump wrote.

Friday’s roll call stood as the most momentous vote to date for Trump and for House Speaker Paul Ryan, R-Wis., both aware that conservatives comprising the heart of their party’s constituency were demanding no less than an all-out assault on Obama’s law.

For Trump, victory would clear an initial but crucial hurdle toward achieving the GOP’s lodestar quest to repeal “Obamacare,” the former president’s 2010 health care overhaul. Defeat could weaken Trump’s political potency by adding a legislative failure to a resume already saddled with inquiries into his campaign’s Russia connections and his unfounded wiretapping allegations against Obama.

In an embarrassing setback Thursday, leaders abruptly postponed the vote because a rebellion by conservatives and moderates would have doomed the measure. They’d hoped for a roll call Thursday, which marked the seventh anniversary of Obama’s enactment of his landmark health care statute that Republicans have vowed ever since to annul.

The leaders seem to be calculating that at crunch time enough dissidents will decide against sabotaging the bill, Trump’s young presidency and the House GOP leadership’s ability to set the agenda, with a single, crushing defeat.

“The president has said he wants the vote tomorrow,” White House budget chief Mick Mulvaney told the lawmakers, according to Rep. Chris Collins, R-N.Y., a Trump ally. “If for any reason it goes down, we’re just going to move forward with additional parts of his agenda. This is our moment in time.”

Even if they prevail, Republicans face an uphill climb in the Senate, where conservatives and moderates are also threatening to sink the legislation.

The GOP bill would eliminate the Obama statute’s unpopular fines on those who do not obtain coverage and the often generous subsidies for those who purchase insurance.

Instead, consumers would face a 30 percent premium penalty if they let coverage lapse. Republican tax credits would be based on age, not income. The bill would also end Obama’s Medicaid expansion and trim future federal financing for the federal-state program and let states impose work requirements on some of its 70 million beneficiaries.