The House is expected to hold a cliffhanger vote on a repeal bill Thursday as acrimony among GOP lawmakers threatens to snatch defeat from the jaws of victory on an objective that has motivated conservative voters for years.

Lawmakers worked late into the night Wednesday but were unable to secure a deal to ensure the bill's passage. President Donald Trump tweeted out a rallying call to supporters on Thursday morning, telling them they were told "many lies" about Obamacare and calling on them to pressure their congressional representatives to back his measure.

"Go with our plan. It's going to be terrific. You're going to be very, very happy," the President said.

The vote heralds a moment of massive political significance, especially for Trump -- who is desperate for a win after a torrid first two months in power and has been working around the clock to seal a deal -- and House Republicans trying to find a way to keep the conservative and moderate coalitions together.

Failure would inflict a serious new blow on the White House in the first genuine test of Trump's capacity to drive his populist agenda through Congress. It would also severely compromise his self-fashioned image as the ultimate deal-maker.

House Speaker Paul Ryan's reputation and capacity to control his splintered GOP caucus is facing its toughest test as well -- and if the bill goes down, he would likely face speculation about how long he can survive in the speaker's chair.

But there are also big upsides for the President and the speaker if they succeed in handing off the Obamacare repeal baton to Senate Majority Leader Mitch McConnell in the Senate.

Trump would finally register a significant legislative win -- something he still lacks nearly two-thirds of the way through his First 100 Days. A victory on health care, along with the likely confirmation of his Supreme Court pick Neil Gorsuch in the coming weeks, would boost a White House that is increasingly under siege.

Ryan would take a tangible step toward becoming the speaker who finally masterminded the repeal of Obamacare -- a significant legacy for the Wisconsin conservative. He would also be free to pivot to policy issues close to his heart, including another Trump priority -- tax reform.

And Trump and Ryan would also have a joint achievement that could cement their so-far tenuous political alliance.

But on the flip side, if the effort collapses, it will unleash a blame game with the potential to open schisms in the GOP's governing majority between Trump and the establishment on Capitol Hill.

Both men used persuasion and threats in the tense hours running up to the vote, focusing especially on the members of the ultra-conservative House Freedom Caucus who believe the "American Health Care Act" does not go far enough to eradicate former President Barack Obama's prized domestic political achievement. But each concession made to the conservatives made the votes of moderate Republicans who are beginning to peel away from the bill even more elusive.

Ryan probably cannot afford to lose more than 21 members of his own caucus to ensure victory. According to CNN's latest whip count Wednesday night, 24 House Republicans have said they will vote against the bill, while four more have indicated they are likely to oppose it, leaving the fate of the measure up in the air.

Trump was piling fierce pressure on holdout lawmakers late Wednesday night in a blitz of telephone calls, as Ryan's negotiators tried to woo the Freedom Caucus with changes to the bill. One proposal would pull the list of essential health benefits , like maternity and pediatric care and prescription drug coverage, out of the measure -- a move that would risk making it even more unpopular with moderates.

Of the two camps, conservatives appeared to be more optimistic about cutting a deal with the White House. Chances of the moderates helping the bill over the line began to fade when Charlie Dent, leader of the Tuesday Group of lawmakers, said he couldn't vote yes.

Asked by a reporter if he plans to keep pressing ahead on health care if the House bill fails, the President simply said on Wednesday: "We'll see what happens."

Surveying the dramatic deal-making taking place on Wednesday, GOP Rep. Tom Cole told CNN: "There's a certain amount of gamesmanship going on here -- a little bit of chicken."

But as they work to finally jam the bill through the House, GOP leaders and the White House say that though they are hopeful, there is no Plan B.

"We've got a promise to keep. We promised the American people we would repeal and replace this law. We have to do it for real, not for fake, for real," Ryan told conservative talk show host Hugh Hewitt on Wednesday.

Trump told Republican lawmakers on Capitol Hill this week that they could lose their seats and GOP majority if they fail to repeal Obamacare. Ryan has praised the President's political engagement.

But even if the two uneasy allies manage to muscle the bill through, their political payoff will be short-lived because the ideological gymnastics needed to thread a path through the House threaten to make it unpalatable in the Senate.

Because of Senate rules and the threat of a Democratic filibuster, Republican leaders have been forced to use a cumbersome three-step formula to pass the bill -- that threatens to stifle its momentum once it reaches the Senate.

They are using a mechanism known as reconciliation -- which can only be used on legislation pertinent to the deficit -- to repeal Obamacare in the Senate. Then Health and Human Services Secretary Tom Price will gut the regulatory framework of Obamacare. Ultimately, Republicans will try to pass a replacement -- a process that will need the votes of Democrats who have little incentive to cooperate in the dismantling of the former President's proudest achievement.

What to do?

In many ways, Republicans face an insoluble dilemma.

On the one hand, they have run for years and won the support of conservative voters on a vow to repeal Obamacare. They face an intense backlash and likely primary challenges next year if they fail to do so.

But uncertainty over a replacement and a Congressional Budget Office estimate that 24 million Americans could end up without health insurance mean that there may be a heavy price to pay for action.

Republicans are vowing to retain the most popular parts of Obamacare, including the ban on insurance companies denying coverage to those with pre-existing conditions, and want to replace the subsidies offered under Obamacare with tax breaks. But some Republicans, particularly state governors, fear that plans to curtail the expansion of Medicaid could deprive millions of people of coverage.

House Republican moderates, who were also at the White House this week, meanwhile fear they will be targeted by Democrats in the mid-terms next year -- and believe the GOP bill could mean higher costs for older constituents.

"I'm a no," GOP Rep. Leonard Lance told reporters after returning from a meeting with Vice President Mike Pence.

Democrats argue that Republicans' promise to provide people with "access" to health care is a code word for actually denying them guaranteed coverage. Trump appears vulnerable in this area because he promised cheaper health care for everyone in his election campaign last year and many of his supporters rely on the Medicaid expansion.

That's one reason the bill could face such heavy going in the Senate. Another is that GOP senators appear to have little patience for the fast-track strategy that Ryan has used to build momentum for the House version of the bill. The Senate is likely to move at a more stately pace, with a series of committee hearings and a flurry of amendments that will in effect rewrite any bill that is sent across from the House.

"I'm not going to vote on this bill until I can understand it, ask questions about it and maybe change it," South Carolina Sen. Lindsey Graham told CNN's Wolf Blitzer on Wednesday.

"The process we're engaging in, I don't think is our best effort to repeal and replace Obamacare."