WASHINGTON (AP) — The GOP's long-promised legislation to repeal and replace "Obamacare" stood on the brink just hours before Republican leaders planned to put it on the House floor for a showdown vote. Short of support, GOP leaders looked to President Donald Trump to close the deal with a crucial bloc of conservatives, in the first major legislative test of his young presidency.

Trump made a final pitch to the public Thursday. In a video posted on his official presidential Twitter account, he asked people to call their lawmakers in support of the legislation.

"Americans were told Obamacare would bring down prices and increase options. You were told that you could keep your plan and keep your doctor," Trump said. "You were given many, many false stories, the fact is you were given many lies. Go with our plan, it's going to be terrific."

The stakes could hardly be higher for a party that gained monopoly control of Washington largely on promises to get rid of former President Barack Obama's signature legislative achievement and replace it with something better. Now Republicans are staring at the possibility of failure at the very moment of truth, an outcome that would be a crushing political defeat for Trump and Hill GOP leaders and would throw prospects for other legislative achievements into extreme uncertainty.

Frenzied last-minute wheeling and dealing was under way on Capitol Hill and the White House, where Trump summoned the balky conservative Freedom Caucus to meet with him mid-day Thursday, ahead of the planned vote. But concessions being offered to the conservatives, who sought to limit requirements for health plans to offer certain benefits including substance abuse and maternity care, appeared to be scaring off moderate Republicans.

The Republican legislation would halt Obama's tax penalties against people who don't 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 the aid 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.

Tension has been building in advance of the critical vote, and a late-night meeting of moderate-leaning members in Speaker Paul Ryan's office Wednesday broke up without resolution or a deal as most lawmakers and Ryan himself left out of side exits and avoided talking to reporters.

One lawmaker present, GOP Rep. Ryan Costello of Pennsylvania, said members had been asked to weigh in on the changes being offered to the Freedom Caucus. He demurred on how the concessions might impact his vote, but said, "The Freedom Caucus has presented what it will take for them to make some 'yeses' and I think there are a lot of members who will now have to evaluate things a little bit further."

Shortly thereafter a key moderate who had been in the meeting, Rep. Charlie Dent of Pennsylvania, issued a statement saying he would be voting "no" on the health bill. "I believe this bill, in its current form, will lead to the loss of coverage and make insurance unaffordable for too many Americans," said Dent, a leader of the Tuesday Group of moderate-leaning Republicans.

Yet Freedom Caucus members, even after winning the prospect of eliminating the so-called "essential health benefits," were not quite ready to guarantee their support.

"Tonight is an encouraging night," Rep. Mark Meadows, R-N.C., leader of the Freedom Caucus, said after a late round of negotiating Wednesday. But after boasting for days he has the votes to kill the health bill, Meadows added that "I don't want to be so optimistic as to say the deal is done."

Thursday's White House meeting between Trump and Freedom Caucus members looked to be pivotal as to whether the vote could go forward as planned. Trump has played an increasingly central role, repeatedly summoning different groups of lawmakers to the White House, traveling to the Capitol himself, and calling others on the phone to press for "yes" votes.

And congressional leaders have increasingly put the onus on the president to close the deal, seemingly seeking to ensure that he takes ownership of the legislation — and with it, ownership of defeat if that is the outcome.

In a count by The Associated Press, at least 26 Republicans said they opposed the bill, enough to narrowly defeat the measure. The number was in constant flux amid the eleventh-hour lobbying.

Including vacancies and expected absentees, the bill would be defeated if 23 Republicans join all Democrats in voting "no."

In a show of support for the opponents, the conservative Koch network promised Wednesday night to spend millions of dollars to help House members who vote against the health care bill.

Moderates were daunted by projections of 24 million Americans losing coverage in a decade and higher out-of-pocket costs for many low-income and older people, as predicted by the nonpartisan Congressional Budget Office.

Yet some Republicans were showing irritation at their party's holdouts.

"At some point we have to cowboy up and prove we can govern," said Rep. Kevin Cramer, R-N.D. "Otherwise we're just going to be the 'no' party and some people are OK with that, it appears."

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Associated Press reporters Alan Fram, Kevin Freking and Ken Thomas contributed to this report.