Sen. John McCain announced his opposition Friday to Republican colleagues’ last-ditch ObamaCare overhaul bill, dealing a major blow to GOP leaders’ push to pass repeal legislation under President Trump.

“I cannot in good conscience vote for the Graham-Cassidy proposal,” the Arizona Republican senator said in a statement, using the bill’s nickname.

McCain – who helped kill the so-called “skinny repeal” back in July – once again was considered a key vote on the current bill, as one of just three wavering GOP senators.

His opposition could effectively doom the bill, which Senate leaders were hoping to bring to the floor next week ahead of a looming deadline. If Republicans cannot pass a bill in this window, it could also mark the end, for the foreseeable future, of the party’s broader efforts to roll back former President Barack Obama’s signature health care law.

WHAT IS GRAHAM-CASSIDY?

McCain announced his decision in a lengthy written statement, warning the bill from Republican Sens. Lindsey Graham of South Carolina and Bill Cassidy of Louisiana was being rushed.

“I would consider supporting legislation similar to that offered by my friends Senators Graham and Cassidy were it the product of extensive hearings, debate and amendment. But that has not been the case,” he said. “We should not be content to pass health care legislation on a party-line basis, as Democrats did when they rammed Obamacare through Congress in 2009. … I cannot in good conscience vote for the Graham-Cassidy proposal.”

With his statement, McCain joins Sen. Rand Paul, R-Ky., who before Friday was the only Republican who had come out firmly against the bill, arguing it doesn’t do enough to roll back ObamaCare.

Senate Republicans hold 52 seats, so McCain’s opposition does not outright kill the legislation.

But the push is precariously close to collapsing. The position of two other Republican senators, Susan Collins of Maine and Lisa Murkowski of Alaska, is not yet firm – but Collins said Friday she’s leaning against the bill.

On Friday evening, Murkowski said she was still undecided.

Unless both senators come out in support of the plan – or Senate Majority Leader Mitch McConnell, R-Ky., can pluck off a Democratic vote or two – Republicans will not have the votes in the Senate.

Senate Democratic Leader Chuck Schumer, D-N.Y., praised McCain on Friday.

“John McCain shows the same courage in Congress that he showed when he was a naval aviator. I have assured Senator McCain that as soon as repeal is off the table, we Democrats are intent on resuming the bipartisan process,” he said in a statement.

Graham, who is friends with McCain, is not giving up hope. In a statement, he said he “respectfully” disagrees with the Arizona senator, calling his bill the “best chance to repeal and replace Obamacare.”

“I’m completely convinced taking money and power out of Washington and returning it to states to administer health care is the best way to replace a collapsing Obamacare system. I’m excited about solutions we have found in Graham-Cassidy-Heller-Johnson. We press on,” he said.

Trump, meanwhile, has fought hard to rally support for the Graham-Cassidy bill, tweeting a warning to Republican foes on Friday morning that the fate of the Affordable Care Act is in their hands.

“Rand Paul, or whoever votes against Hcare Bill, will forever (future political campaigns) be known as ‘the Republican who saved ObamaCare,’” Trump tweeted.

The reason Republicans were moving quickly to pass the bill is because they face a Sept. 30 deadline, after which special rules that prevent a Democratic filibuster will expire.

Democrats are unanimously opposed.

The Graham-Cassidy bill would repeal major pillars of Obama's law, replacing them with block grants to states to design their own programs. Major medical groups are opposed, saying millions would lose insurance coverage and protections.

The Associated Press contributed to this report.