The House will vote Thursday on a bill to repeal and replace ObamaCare, a GOP source said.

The announcement sets up a high-stakes vote that is expected to come down to the wire.

The move comes after the revised American Health Care Act gained new momentum on Wednesday, when GOP Reps. Fred Upton (Mich.) and Billy Long (Mo.) said they would support the bill after a new amendment from Upton.

The amendment released Wednesday night listed as co-sponsors four lawmakers who were either undecided or had previously planned to vote no on the bill.

ADVERTISEMENT It's unclear if their co-sponsorship means Reps. David Valadao (Calif.), Jeff Denham (Calif.), Steve Knight (Calif.) and David Young (Iowa) are now willing to vote yes.

Just two other lawmaker have publicly switched his stance since the announcement of the new amendment — Reps. Daniel Webster (Fla.) and Ted Budd (N.C.)

Asked by a reporter about whether the bill would have to be pulled from the floor again for lack of support, McCarthy replied: "Would you have confidence? We're going to pass it. We're going to pass it. Let's be optimistic about life." McCarthy cited an insurer pulling out of the ObamaCare exchanges in Iowa Wednesday as a reason the law needs to be quickly repealed. "That's why we have to make sure this passes. To save these people from ObamaCare, which continues to collapse." "We will be voting on the healthcare bill tomorrow. We have enough votes. It will pass. It's a good bill," House Majority Leader Kevin McCarthy (R-Calif.) said after leaving a leadership meeting Wednesday night.

The Upton amendment would provide $8 billion over five years to help people with pre-existing conditions afford their premiums in states that are granted a waiver from ObamaCare’s community rating provision, which prevents insurers from charging premiums based on customers' health.

The House will vote without waiting for a new Congressional Budget Office (CBO) analysis of Upton's changes or the amendment from Rep. Tom MacArthur (R-N.J.) that won over the conservative House Freedom Caucus. That analysis will eventually provide insight into the bill's effects on coverage and its cost.

The MacArthur amendment set off moderates' concerns, because it allows states to waive ObamaCare community rating. In states receiving that waiver, insurers could go back to charging exorbitant premiums to sick people, which could put coverage out of reach for many.

Supporters of the amended bill argue high-risk pools, which help people with high medical costs afford insurance, could fill the gap and note that people with pre-existing conditions would still be protected if they had no gaps in coverage.

Many health policy experts, though, said the additional $8 billion from the Upton amendment is not enough to help sick people get coverage.

The liberal Center for American Progress estimated on Tuesday that the high-risk pools are underfunded by $200 billion over 10 years.

Members of the moderate Tuesday Group discussed the changes at a Wednesday meeting that included House Energy and Commerce Committee Chairman Greg Walden (R-Ore.).

Leaving that meeting, no lawmaker said they had changed their position.

Rep. John Faso (R-N.Y.) said he was still undecided and studying the changes. Rep. Adam Kinzinger (R-Ill.) declined to comment when asked if he was still undecided.

The underlying bill has several provisions some moderates object to, such as deep Medicaid cuts. The CBO estimated an earlier version of the overall legislation would result in 24 million more people becoming uninsured over a decade.

Major health groups like the American Medical Association and the American Hospital Association are strongly opposed to the measure, warning of coverage losses and other problems.

According to the Hill's whip list, there are 18 no votes from the GOP on the latest iteration of the bill. No Democrats are expected to vote for the measure, meaning Republicans can only afford 22 defections.

Updated 10:23 p.m.