– The latest Republican effort to dismantle the Affordable Care Act stood on the brink of failure Friday after Sen. John McCain, R-Ariz., announced his opposition to the proposal and Sen. Susan Collins, R-Maine, said that she was leaning against it.

The intensifying resistance dealt a potentially decisive blow to the renewed attempt to fulfill a seven-year-old GOP promise. McCain joined Sen. Rand Paul, R-Ky., in formally opposing the plan, leaving party leaders one senator away from defeat.

“That was a totally unexpected thing,” President Donald Trump said of McCain’s decision at a campaign rally Friday for Republican Sen. Luther Strange in Huntsville, Ala. “Terrible. Honestly, terrible.”

Friday’s developments forced Senate Majority Leader Mitch McConnell, R-Ky., and Trump into a corner. They must now decide whether to continue to pursue a vote that increasingly appears likely to fail, or end the endeavor and deal with the backlash after another unsuccessful try.

Another GOP failure to undo the ACA could have a seismic impact on the legislative dynamic in Washington and the emerging contours of the 2018 midterm elections.

Trump’s relationship with McConnell has grown sour since an earlier failure to repeal the law over the summer and the current push represents a chance to repair that relationship. If it fails, Trump could turn on congressional Republicans more forcefully and be tempted to work with Democrats, whom he has courted on a series of narrower issues.

One overriding obstacle for Republicans is that their efforts to roll back the ACA are deeply unpopular among the broader public. A new Washington Post-ABC News poll released Friday showed that more than half of Americans (56 percent) prefer the ACA to the latest GOP plan. Only 33 percent prefer the Senate Republicans’ legislation.

In a lengthy written statement Friday, McCain said he “cannot in good conscience” vote for the bill authored by Sens. Bill Cassidy, R-La., and Lindsey Graham, R-S.C., which GOP leaders had planned to bring to the Senate floor next week. He railed against the hurried process leaders have used to move the bill ahead.

“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,” McCain said. He blamed a looming Sept. 30 deadline to take advantage of a rule allowing Republicans to pass the bill with as few as 50 Senate votes, plus Vice President Mike Pence as a tiebreaker.

Senate Republicans hold a narrow 52-48 majority and Democratic senators are united against repealing or gutting President Barack Obama’s signature health care law.

McCain also said he could not vote for a bill without a complete snapshot of its effects from the nonpartisan Congressional Budget Office, which said this week it could only provide a partial picture by next week. The office said it could not determine the bill’s impact on insurance premiums or project the change in insurance coverage levels it would trigger until a later date.

“I believe we could do better working together, Republicans and Democrats, and have not yet really tried,” McCain said. He added that he took “no pleasure” in his decision.

In her home state on Friday, Collins also signaled that she is edging close to becoming a definite “no.” Like McCain, Collins voted against a different GOP repeal bill in July that was rejected by the Senate.

“I’m leaning against the bill,” she said in Portland, Maine. “I’m just trying to do what I believe is the right thing for the people of Maine.”

Collins has said that she is particularly worried that by giving states wide latitude to change the ACA’s current requirements, it could prompt insurers to hike up rates for consumers with costly medical conditions. “I’m reading the fine print,” she said.

Another wild card, Sen. Lisa Murkowski, R-Alaska, was still undecided late Friday, according to her communications director, Karina Petersen. “Right now she is still focused on how the bill will impact Alaska,” Petersen said.

Paul spokesman Sergio Gor reiterated his boss’s opposition to Cassidy-Graham on Friday, after Trump threatened Paul and other senators on Twitter.

“Rand Paul, or whoever votes against Hcare Bill, will forever (future political campaigns) be known as ‘the Republican who saved ObamaCare,” Trump tweeted. Paul replied in tweets saying he “won’t be bribed or bullied.”

Graham said Friday that he planned to continue trying to bring the plan to a vote.

But some were skeptical the rebooted effort could continue. At a town hall in liberal Iowa City, which began an hour after McCain announced his opposition, Sen. Joni Ernst, R-Iowa, told a cheering crowd stacked with ACA supporters that the GOP’s repeal push was likely over for the year.

“I’ll be honest,” Ernst said. “It seems unlikely that we’ll be voting on this.”

Cassidy-Graham would turn funding for the ACA into block grants for states and sharply cut Medicaid spending over time. Three independent analyses and an internal one from the Trump administration have all predicted that more than 30 states would lose federal funding between 2020 and 2026 under the measure.

Broadly speaking, states with low health care costs that provide fewer Medicaid benefits and failed to expand the program under the ACA stand to gain under the bill, while others stand to lose.

While it is difficult to calculate how the Graham-Cassidy bill would affect the number of Americans with health insurance, an analysis published Friday by the Brookings Institution and the University of California Schaeffer Center for Health Policy & Economics projected that roughly 15 million Americans would lose coverage over the next two years if the bill was enacted.

Powerful groups representing insurers and physicians have come out forcefully against the bill this week. Patient advocacy groups have been mobilizing in force to try to sway members who have yet to say how they will vote.

On Monday, a coalition of 20 groups plan a rally opposing the bill at the U.S. Capitol.