WATCH LIVE: House of Representatives votes on GOP's health care bill

Republicans under pressure to give President Donald Trump a legislative victory could take a dramatic step toward that end Thursday when the US House votes on dismantling Barack Obama's signature health reforms.

WATCH LIVE VIDEO OF THE HOUSE FLOOR BELOW:

"We have enough votes," Majority Leader Kevin McCarthy told reporters late Wednesday. "It'll pass. It's a good bill."

The announcement came as a revised health care reform bill gained momentum when two influential Republican representatives -- Fred Upton of Michigan and Billy Long of Missouri -- reversed course Wednesday and threw their support behind the measure following intense lobbying by the president himself.

The vote, which follows weeks of tweaking to the controversial bill in order to bring enough Republicans on board, is expected to be close.

With no Democrats supporting the revision, Republicans alone will need to cobble together the 216 votes necessary to pass the legislation, despite internal differences on the way forward. They can afford just 22 defectors.

The effort to repeal and replace the Affordable Care Act has been seven years in the making. Republicans including Trump have campaigned relentlessly on pledges to dismantle Obamacare, and Thursday's vote is the closest they have come since Trump won the White House.

An earlier version of the Republican plan collapsed in March, when opposition from both moderates and conservatives torpedoed their own party's attempt to do away with former president Barack Obama's 2010 law.

But leadership apparently won over enough skeptical members with an amendment, drafted by Upton, which adds $8 billion to help cover insurance costs for people with pre-existing conditions.

Trump has touted the amended draft multiple times in recent days, and engaged personally in the arm-twisting, reaching out to several Republicans by phone and face to face.

"The president said 'Billy we really need you. We need you, man.' I said 'You don't have me,'" Long told reporters at the White House, describing an extended back-and-forth with Trump over what it would take for Long to back the bill.

Long says he jumped on board after the president gave his blessing to the amendment that adds the supplemental $8 billion, which would be used to help fund so-called "high-risk pools" aimed at absorbing some of the costs for people with expensive conditions, like cancer.

- 'Not a panacea' -

Upton previously opposed the legislation, saying he was uncomfortable with a provision allowing states to remove coverage guarantees for people with pre-existing conditions -- guarantees that were provided under Obamacare.

But after meeting Trump, Upton said he felt that the new funding under his amendment would "more than cover those who might be impacted."

Many health policy experts, however, say $8 billion -- spread over five years -- is far from enough to help provide coverage for sick people.

High-risk pools were used by several states prior to Obamacare's passage, and they were "not a panacea for Americans with pre-existing medical conditions," said American Medical Association president Andrew Gurman in a statement opposing the latest Republican bill.

"Not only would the AHCA (the Republicans' American Health Care Act) eliminate health insurance coverage for millions of Americans, the legislation would, in many cases, eliminate the ban against charging those with underlying medical conditions vastly more for their coverage," he added.

"This craven bill is going to hurt many, many more Americans," added Democratic Senator Cory Booker on MSNBC.

The bill faces a steeper path in the Senate, where Republican moderates are almost certain to introduce changes to the legislation in order to get it passed.

That would trigger another round of negotiations between House and Senate members in a bid to get the bill over the line -- a result that remains far from certain.

Meanwhile, Republicans are heading to a health care vote without a new estimate of what the AHCA would cost.

An earlier non-partisan congressional review in March predicted that under the Republican plan, 24 million more people would be uninsured by 2026 than under the current law.

The United States is the only major developed country lacking national health care for all.