WASHINGTON >> John McCain seemed poised to be the savior of the GOP health bill when he returned to the Capitol despite a brain cancer diagnosis.

He turned out to be the executioner.

The longtime Arizona senator stunned pretty much everyone Friday by turning on his party and his president and joining two other GOP senators in voting “no” on the Republicans’ final effort to repeal “Obamacare.”

That killed the bill. And it also dealt what looks like a death blow to the Republican Party’s years of promises to get rid of Barack Obama’s health law, pledges that helped the GOP win control of the House, the Senate and the White House.

It was a moment burning with drama, irony and contradictions, playing out live on a tense Senate floor.

Eighty years old and in the twilight of a remarkable career, McCain lived up to his reputation as a maverick. When he walked into the well of the Senate around 1:30 a.m. and gave a thumbs-down to the legislation, there were audible gasps. Democrats briefly broke into cheers, which Minority Leader Chuck Schumer quickly waved his arm to quiet.

Majority Leader Mitch McConnell stood stone-faced, his arms crossed. McCain had just saved the signature legislative achievement of the man who beat him for the presidency in 2008, a law the senator himself had vigorously campaigned against while seeking a sixth Senate term last year.

Friday afternoon, McCain’s office announced he was returning to Arizona to begin radiation and chemotherapy treatments for his brain tumor.

After so many years as a senator, with so little left to lose, McCain had taken a stand for the Senate he used to inhabit, the one where he made deals across the aisle with the likes of Ted Kennedy, not the riven, stalemated Congress of today.

“We have seen the world’s greatest deliberative body succumb to partisan rancor and gridlock,” McCain said in a statement. “The vote last night presents the Senate with an opportunity to start fresh. It is now time to return to regular order with input from all of our members — Republicans and Democrats — and bring a bill to the floor of the Senate for amendment and debate.”

President Donald Trump tweeted his disapproval of McCain’s “no’” vote, as well as those of fellow GOP Sens. Susan Collins of Maine and Lisa Murkowski of Alaska whose opposition had been expected. But a president who once mocked McCain’s years as a prisoner of war in Vietnam did not have much sway with the senator when it counted.

“John McCain is blessed with an internal gyroscope of right and wrong,” said Schumer, who negotiated a sweeping immigration bill with McCain several years ago and has been talking with him frequently of late. “He gets angry, for sure, but when push comes to shove and there are brass tacks, that internal gyroscope of right and wrong guides him.”

Vice President Mike Pence lobbied McCain right up to the end. The two men huddled on the Senate floor for about a half hour before the vote.

As their conversation ended, McCain and Pence smiled and patted each other on the back, and McCain walked across the floor to talk with Schumer. About a dozen Democrats gathered around him. McCain held out his hands, looked upward and mouthed an expletive. His face looked exasperated.

And then, as Democratic Sen. Chris Murphy of Connecticut described it later in a post on the website Medium, “Time seems to stand still.”

The roll was called, and Collins and Murkowski both voted no. With Democrats unanimously opposed, McConnell could lose only two Republicans in the 52-48 Senate.

Finally McCain came to the front, raised his arm to get the attention of the tally clerk, gestured no, and walked away past the glowering McConnell. With that one moment, seven years of urgent GOP promises were dead, likely never to be revived.

McConnell’s remarks in the immediate aftermath were a bitter rebuke.

“I and many of my colleagues did as we promised and voted to repeal this failed law,” the majority leader said on the Senate floor. “We told our constituents we would vote that way and when the moment came, when the moment came, most of us did.”

Just days earlier, on Tuesday, McCain had buoyed the efforts of McConnell — and Trump — when he returned to the Capitol for the first time after being diagnosed with a brain tumor, and cast a decisive vote to open debate on the GOP repeal legislation. Yet even then he forecast that his support could not be counted on, as he took the floor to lecture his colleagues, the scars from his surgery etched severely along the left side of his face.

“Why don’t we try the old way of legislating in the Senate, the way our rules and customs encourage us to act,” he said. “If this process ends in failure, which seems likely, then let’s return to regular order.”

The outcome McCain predicted came to pass — he made sure that it did. And now if Republicans want to get anything done on health care, they will have little choice but to return to regular order, and turn to Democrats.