WASHINGTON - US President Donald Trump's bid to repeal his predecessor's signature health care law suffered a bitter blow Thursday, as opposition from within his Republican Party forced the delay of a crucial vote in Congress.

"No vote tonight," a House leadership source told AFP, signaling a stunning political setback for Trump -- who prides himself on his deal-making skills -- to win sufficient support for a Republican bill repealing and replacing Obamacare.

The president and his lieutenants repeatedly voiced optimism about the bill's prospects and said they had made progress in convincing doubters to join his camp in dismantling the Affordable Care Act.

But the votes for Trump's plan -- dubbed the American Health Care Act -- weren't there.

"I am still a no at this time. I am desperately trying to get to yes," said Mark Meadows, chairman of the conservative House Freedom Caucus, whose members have demanded major changes to the plan before giving their blessing.

While Meadows sought to portray optimism about the process, he revealed the width of the gap between Trump and plan opponents.

"At this point, we are trying to get another 30 to 40 votes that are currently in the 'no' category to 'yes,'" Meadows said after meeting with his caucus.

"Once we do that, I think we can move forward to passing it on the House floor."

House Republicans were preparing to head into a closed-door conference at 7 p.m. (2300 GMT) to thrash out their differences and perhaps come to agreement on a way to bring enough Republicans on board.

A White House official said the expectation was for a vote Friday, and downplayed suggestions that Trump had failed to close the deal, claiming the delay did not spell doom for the measure.

"The vote will be in the morning to avoid voting at 3 am," the official said.

"We feel this should be done in the light of day, not in the wee hours of the night and we are confident the bill will pass in the morning," the official said.

That schedule was reiterated by House Majority Leader Kevin McCarthy after the vote's postponement.

But with negotiations unable to provide the necessary breakthrough, nighttime debate is likely to be intense as the two sides seek an elusive compromise.

Failure to work out their differences would mark a humiliating defeat for Trump's biggest legislative battle to date.

DIVISION

Republicans have spent years railing against the Affordable Care Act, branding it an example of Democrats pushing for socialized medicine.

But seven years to the day since Barack Obama signed his landmark reforms into law, House Republican leaders were unable to present a united front within their own ranks for the alternative.

With Democrats opposed to Trump's effort to rip out his predecessor's crowning domestic achievement, and his own party's right flank in revolt, the White House and Republican leaders have been burning the midnight oil to find ways to make the bill palatable to enough conservatives without angering moderates.

Confidence by the White House appeared to highlight the disconnect between Trump's team and rank-and-file conservatives.

Asked Thursday whether House Speaker Paul Ryan might delay the vote, White House spokesman Sean Spicer said "nothing leads me to believe that that's the case."

A few hours later, the vote was postponed.

Many conservatives say their party's plan is still too costly for the government.

They have said they want to repeal health benefits that all insurance policies must pay for under Obamacare -- including maternity care, emergency room visits, and preventive care like screenings and vaccines -- arguing they have driven up costs.

LIMITING DEFECTIONS

The House Freedom Caucus, about 30 lawmakers who are heirs apparent to the ultra-conservative Tea Party movement, have dubbed the new bill "Obamacare Lite," as it will only reduce, not eliminate, health coverage subsidies by replacing them with refundable tax credits.

At the other end of the spectrum, some Republican moderates also worry their constituents would no longer be able to afford health insurance under the new plan.

A nonpartisan congressional budget estimate says it would lead 14 million Americans to lose their coverage from next year.

The Democratic minority is prepared to vote against it as a bloc, so Republican leaders need to limit defections to fewer than 22 out of their party's 237 representatives among the House's 430 current members.

Further pressuring recalcitrant Republicans, Trump tweeted out messages to his tens of millions of followers urging them to contact their local lawmakers in support of the plan.

Congressman Thomas Massie said the arm-twisting would not work on him.

"I'm still opposed to the bill," the Kentucky Republican told MSNBC. "I think it's worse than Obamacare."

Obama himself weighed in Thursday on the law's anniversary, saying the reform that has helped 20 million people get coverage should be improved, not pulled out by its roots.

"We should start from the baseline that any changes will make our health care system better, not worse for hardworking Americans. That should always be our priority," Obama said in a statement.