Sen. John McCain (R-Ariz.) said Friday that he could not vote for the GOP’s latest health-care bill because the legislative process was too rushed. (Melina Mara/The Washington Post)

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 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.

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

A fresh GOP failure to undo Obamacare 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 attempt 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.

Many Republicans fear that a defeat could also depress the GOP base headed into the midterms, potentially reducing turnout next fall and creating an environment in which GOP incumbents are ripe for primary challenges from angry conservatives.

Senate Republicans are trying to revive the momentum to overhaul the Affordable Care Act with the Cassidy-Graham proposal. Here are five things to know about the plan and the rush to pass it. (Jenny Starrs/The Washington Post)

One overriding obstacle for Republicans, however, 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 bill that Senate Republicans abruptly put on the table this month.

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 O. Graham (R-S.C.), which GOP leaders have been aiming to bring to the Senate floor next week. As he has done repeatedly in recent days, he railed against the hurried process leaders have used to move the measure 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 procedural rule allowing Republicans to pass the bill with as few as 50 Senate votes, plus Vice President Pence as a tiebreaker.

Senate Republicans hold a narrow 52-to-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 earlier this week that it could provide only 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 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 announcement. McCain and Graham are close friends.

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

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

Collins has said she is particularly worried that giving states wide latitude to change the ACA’s requirements could prompt insurers to increase rates for consumers with costly medical conditions.

“I’m reading the fine print,” she said, adding that for those with preexisting conditions, “their premium could be so high that it would not be affordable.”

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 — who objects to the legislation because it does not fully repeal the ACA — responded in a series of tweets, saying that he “won’t be bribed or bullied” into changing his mind.

Graham said Friday that he planned to continue trying to bring the measure to a vote. McConnell’s office, which said earlier that he intended to bring the bill to the Senate floor next week, did not respond to an inquiry about what he plans to do next.

But some were skeptical that the rebooted effort could continue.

At a town hall in liberal Iowa City, which began an hour after McCain announced his opposition to the bill, Sen. Joni Ernst (R-Iowa) told a cheering crowd stacked with ACA supporters that the GOP’s repeal push was probably 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 analysis 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 did not expand the program under the ACA stand to gain under Cassidy-Graham, while others stand to lose. As a result, states such as Texas, Georgia and Mississippi would get more funding, while California, New York and Maryland would take hits.

While it is difficult to calculate how the Cassidy-Graham bill would affect the number of Americans with health insurance, an analysis published Friday by the Brookings Institution and the University of Southern California’s 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 organizations 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 lawmakers who have yet to say how they will vote. A coalition of 20 groups is planning to hold a rally Monday at the Capitol, where the featured speakers will include a lung cancer patient, and a mother and her 14-year-old son who was born with congenital heart disease.

Senate Republican leaders and Trump administration officials have continued to lobby for the measure. Pence, who hosted Maine Gov. Paul LePage (R) at his ceremonial office Friday, said the Cassidy-Graham bill “is an idea whose time has come.”

“President Trump and I are absolutely determined to carry this case all across the country and to call on members of the Senate — most especially Senator Susan Collins from the great state of Maine — to join us in giving the people of Maine and the people of America a fresh start on health-care reform,” Pence said.

Another wild card, Sen. Lisa Murkowski (R-Alaska), was undecided late Friday, according to her communications director, Karina Petersen. “Right now she is still focused on how the bill will impact Alaska specifically. She’s continuing to gather data and is looking at the details of the bill to determine what’s best for her state,” Petersen said.

Alaska officials published a preliminary analysis Thursday showing that from 2020 to 2026, the state would lose nearly a quarter of its federal funding for Medicaid and private insurance, or $1.1 billion.

Meanwhile, Graham said on Twitter that his “friendship with [McCain] is not based on how he votes but respect for how he’s lived his life and the person he is,” adding that he was “excited about solutions” in the bill.

“We press on,” he concluded.

Senate Democrats praised McCain’s surprise announcement and called for Republicans to resume stalled negotiations between Senate Health, Education, Labor and Pensions Committee Chairman Lamar Alexander (R-Tenn.) and Sen. Patty Murray (Wash.), the panel’s ranking Democrat, on legislation to strengthen existing health insurance marketplaces.

Murray and Alexander appeared to be nearing an agreement earlier in the week on a deal to ensure continued payment of federal subsidies to help reduce out-of-pocket costs and premiums for low-income people. But Alexander abruptly ended the talks as GOP leaders intensified their efforts to win votes for Cassidy-Graham.

Sen. Bernie Sanders (I-Vt.), who flew to California on Friday to speak at the annual conference of National Nurses United, asked activists to join him in thanking McCain. But Sanders also said that the “struggle over this legislation is not over,” urging activists to “do everything you can, get the word out all over this country, to tell them that no Republican should vote for this.”

David Weigel in Iowa City and Amber Phillips and Amy Goldstein in Washington contributed to this report.