President Donald Trump has demanded a make-or-break vote on healthcare legislation in the House, threatening to leave Obamacare in place and move on to other issues if Friday's ballot fails.

The move was presented to Republican lawmakers behind closed doors on Thursday night after a long day that saw a planned vote on the healthcare bill scrapped as the legislation remained short of votes amid negotiations among conservative lawmakers, moderates and others.

At the end of it, the president had had enough and was ready to vote and move on, whatever the result, Trump's budget director Mick Mulvaney told lawmakers.

"'Negotiations are over. We'd like to vote tomorrow and let's get this done for the American people.' That was it," Duncan Hunter of California said as he left the meeting.

"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 hard to predict.

Conservative and moderate lawmakers had claimed the bill lacked votes after a long day of talks but the White House appeared ready to gamble the prospect of failing to repeal former President Barack Obama's health law after seven years of promising to do exactly that, would force lawmakers into the yes column.

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

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

It would provide tax credits to help people pay medical bills, though generally skimpier than 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 federal payments to Planned Parenthood for a year, another stumbling block for GOP moderates