WASHINGTON (AP) — Sen. John McCain declared his opposition Friday to the GOP's last-ditch effort to repeal and replace "Obamacare," dealing a likely death blow to the legislation and, perhaps, to the Republican Party's years of vows to kill the program.

"I cannot in good conscience vote for the Graham-Cassidy proposal," McCain said in a statement, referring to the bill by Sens. Lindsey Graham of South Carolina and Bill Cassidy of Louisiana. His opposition likely leaves the bill at least one vote short of the support needed for passage.

"I believe we could do better working together, Republicans and Democrats, and have not yet really tried," McCain said. "Nor could I support it without knowing how much it will cost, how it will affect insurance premiums, and how many people will be helped or hurt by it."

McCain was the decisive vote against the GOP's last repeal effort, in July. Once again, the 81-year-old senator, battling brain cancer in the twilight of a remarkable career, emerged as the destroyer of his party's signature promise to voters.

President Donald Trump and Senate Majority Leader Mitch McConnell have both been pushing hard for the bill in recent days, and McCain's best friend in the Senate, Graham, was an author. Trump declared during the presidential campaign that he would quickly demolish Obamacare and "it will be easy."

Democrats are unanimously opposed. GOP Sen. Rand Paul of Kentucky has announced his opposition and GOP Sen. Susan Collins of Maine said Friday she, too, was leaning against supporting the bill.

Along with McCain, that would leave Republicans with 49 votes, at most, for the bill. They would need 50, plus Vice President Mike Pence to break a tie, in order to prevail in the 100-member Senate.

GOP leaders had hoped to bring the legislation to the floor next week. They face a Sept. 30 deadline, at which point special rules that prevent a Democratic filibuster will expire.

Democrats hailed McCain's announcement and pledged to commit to the bipartisan process he sought.

"John McCain shows the same courage in Congress that he showed when he was a naval aviator," said Senate Minority Leader Chuck Schumer, D-N.Y. "I have assured Sen. McCain that as soon as repeal is off the table, we Democrats are intent on resuming the bipartisan process."

The Graham-Cassidy bill would repeal major pillars of former President Barack 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, and a bipartisan group of governors also has announced opposition.

Yet Republican congressional leaders, goaded by GOP voters and the president himself, were determined to give it one last try. Trump spent much of August needling McConnell for his failure to pass a repeal bill, and Republican lawmakers back home during Congress' summer recess heard repeatedly from voters angered that after seven years of promises to get rid of "Obamacare," the party had not delivered.

The House passed its own repeal bill back in May, prompting Trump to convene a Rose Garden celebration which soon began to look premature.

After the Senate failed in several attempts in July, the legislation looked dead. But Cassidy kept at it with his state-focused approach, and the effort caught new life in recent weeks as the deadline neared, with Trump, hungry for a win and with evidence mounting that the GOP could face severe political repercussions for failing to deliver on promises that had helped the party win control of the House, Senate and White House.

There was fresh negative attention, too. Late-night TV host Jimmy Kimmel, whose son was born with medical problems, criticized the new repeal effort at length on his show and said the Republicans were lying about what their new program would cover. In a tweet Friday, he thanked McCain "for being a hero again and again and now AGAIN."

This past week McConnell fully embraced the Graham-Cassidy bill for the first time and his office declared his intention to bring it to a vote next week.

Trump tweeted Friday morning that "Rand Paul, or whoever votes against Hcare Bill, will forever (future political campaigns) be known as 'the Republican who saved ObamaCare.'"

The bill would get rid of unpopular mandates for people to carry insurance or face penalties. It would repeal the financing for Obama's health insurance expansion and create a big pot of money states could tap to set up their own programs, with less federal oversight. It would also limit spending for Medicaid, the federal-state program that now covers more than 70 million low income people. Insurance rules that protect people with pre-existing conditions could be loosened through state waivers.

Over time, the legislation would significantly reduce federal health care dollars now flowing to the states.