WASHINGTON • US President Donald Trump has issued an ultimatum to recalcitrant Republicans to fall in line behind a broad health insurance overhaul or see their opportunity to repeal Obamacare vanish, demanding a vote on a Bill that appeared to lack a majority to pass.

A procedural vote on the Bill was set for yesterday morning, followed by a full floor vote in the afternoon.

House leaders were forced to postpone a Thursday vote on the measure amid a revolt by mainly conservative Republicans, who were complicating the first major legislative test for the new president by signalling it would not pass without key changes.

Mr Trump himself set the stage, dispatching an aide to a closed- door meeting of Republican lawmakers to demand a vote.

"The message is tomorrow it's up, it's down - we expect it to be up - but it's done tomorrow," White House budget director Mick Mulvaney told the lawmakers, according to congressman Chris Collins.

Mr Mulvaney then delivered Mr Trump's extraordinary ultimatum.

"If it doesn't pass, we're moving beyond healthcare," Mr Mulvaney said, as paraphrased by Mr Collins. "We would be moving on to other parts of his agenda."

The idea that Mr Trump - who campaigned relentlessly on a pledge to bury Mr Barack Obama's signature legislative accomplishment - would wash his hands of the fight and let Obamacare stand is a startling departure from the party playbook. But Mr Mulvaney's blunt take-it-or-leave-it approach could be part of Mr Trump's hardball strategy to get Republican rebels to fall in line.

"We have a great Bill, and I think we have a good chance, but it's only politics," Mr Trump said earlier on Thursday as it was becoming clear that his negotiating efforts had failed to persuade enough members of his party to back the plan - which was years in the making - to repeal and replace the health law.

He appealed to supporters to weigh in, assuring them in a video on Twitter: "Go with our plan. It's going to be terrific."

Mr Trump spent much of the day lobbying both conservative lawmakers and moderates in a delicate arm-twisting effort.

"Tomorrow it's time to vote," White House spokesman Sean Spicer told Fox News.

"At the end of the day, this is the only train leaving the station that's going to be repealing Obamacare and giving us an alternative to replace it," he said.

Privately, White House officials conceded that competing Republican factions were each demanding changes that could doom the effort, placing the measure in peril and Mr Trump's chances of succeeding at a high-stakes legislative deal in jeopardy.

"We have been promising the American people that we will repeal and replace this broken law because it's collapsing and it's failing families, and tomorrow, we're proceeding," House Speaker Paul Ryan told reporters after an evening all-hands meeting. He ignored those who asked if he had secured the votes to pass the Bill. The Republicans have a majority in the House but, because of united Democratic opposition, can afford to lose only 21 Republican votes.

By Thursday morning, NBC News was saying that 30 Republicans had planned to vote "no" or were leaning that way.

The vote has been seen by financial markets as a crucial test of Mr Trump's ability to work with Congress to deliver on his other priorities, such as tax cuts and infrastructure spending.

Even if the replacement plan does eventually get approval from the House, the legislation faces a potentially tough fight in the Republican-controlled Senate.

NYTIMES, REUTERS, AGENCE FRANCE-PRESSE