US President Donald Trump has abandoned negotiations and demanded a make-or-break decision on healthcare, threatening to leave “Obamacare” in place and move on to other issues if Friday's vote fails.

The risky move, part gamble and part threat, was presented to Republican politicians behind closed doors on Thursday night after a long and intense day that saw a planned vote on the healthcare bill scrapped as the legislation remained short of votes amid cascading negotiations among conservatives, moderates and others.

At the end, the President had had enough and was ready to vote and move on, whatever the result, Mr Trump's budget director Mick Mulvaney told House of Representatives members.

“'Negotiations are over, we'd like to vote tomorrow and let's get this done for the American people'. That was it,” congressman Duncan Hunter of California said as he left the meeting, summarising Mr Mulvaney's message.

“Let's vote,” White House chief strategist Steve Bannon said as he walked out.

“For seven and a half years 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 said, then walked off without answering as reporters demanded to know whether the bill had the votes to pass.

The outcome of Friday's vote is impossible to predict.

Both conservatives and moderates had claimed the bill lacked votes after a long day of talks. But the White House appeared ready to gamble that the prospect of failing to repeal former president Barack Obama's Affordable Care Act health law, after seven years of promising to do exactly that, would force members into the Yes column.

“It's done tomorrow. Or Obamacare stays,” said Chris Collins, a top Trump ally in the House.

Mr Collins was among those predicting success on Friday, but others did not hide their anxiety about the outcome.

Asked whether Republicans would be unified on Friday's vote, freshman congressman Matt Gaetz of Florida said: “I sure hope so or we'll have the opportunity to watch a unified Democratic caucus impeach Donald Trump in two years when we lose the majority.”

Thursday's manoeuvres added up to high drama on Capitol Hill, but Friday promised even more suspense with the prospect of leadership putting a major bill on the floor uncertain about whether it would pass or fail.

The Republican legislation would halt Mr Obama's tax penalties against people who do not buy coverage and cut the government Medicaid programme for low earners, which the Obama statute had expanded.

It would provide tax credits to help people pay medical bills, though generally skimpier than Mr Obama's statute provides.

It also would allow insurers to charge older Americans more and repeal tax boosts the law imposed on high-income people and health industry companies.

The measure would also block national payments to Planned Parenthood for a year, another stumbling block for Republican moderates.

In a concession to the conservative House Freedom Caucus, many of whose members have withheld support, the legislation would repeal requirements for insurers to cover “essential health benefits” such as maternity care and substance abuse treatment.

The drama unfolded seven years to the day after Mr Obama signed his landmark law, an anniversary Republican leaders meant to celebrate with a vote to undo the divisive legislation.

Obamacare gave birth to the tea party movement and helped Republicans win and keep control of Congress and then take the White House.

“In the final analysis, this bill falls short,” Jaime Herrera Beutler of Washington state said as she became the latest rank-and-file Republican, normally loyal to leadership, to declare her opposition.

“The difficulties this bill would create for millions of children were left unaddressed,” she said, citing the unravelling of Medicaid.

In a danger sign for Republicans, a Quinnipiac University poll found that people disapprove of the Republican legislation by 56% to 17%, with 26% undecided.

Mr Trump's handling of healthcare was viewed unfavourably by six in 10.

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House minority leader Nancy Pelosi, who as speaker was Mr Obama's crucial lieutenant in passing the Democratic bill in the first place, could not resist a dig at the Republican disarray.

“You may be a great negotiator,” she said of Mr Trump. “Rookie's error for bringing this up on a day when clearly you're not ready.”

Mr Obama declared in a statement that “America is stronger” because of the current law and said Democrats must make sure “any changes will make our healthcare system better, not worse for hard-working Americans”.

Mr Trump tweeted to supporters: “Go with our plan! Call your Rep & let them know.”