WASHINGTON (AP) -- 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.