WASHINGTON — The Senate voted narrowly on Tuesday to begin debate on a bill to repeal major provisions of the Affordable Care Act, taking a pivotal step forward after the dramatic return of Senator John McCain, who cast a crucial vote despite his diagnosis of brain cancer.
The 51-50 vote, with Vice President Mike Pence breaking a tie, came only a week after the Republican effort to dismantle a pillar of former President Barack Obama’s legacy appeared all but doomed. It marked an initial win for President Trump, who pushed, cajoled and threatened senators over the last days to at least begin debating the repeal of the health care law.
But the victory could be a fleeting one: Senate Republicans still had no agreement on a repeal bill that they could ultimately pass to uproot the law that has provided health insurance to millions of Americans.
The podcast that makes sense of the most delirious stretch of the 2016 campaign.
The Senate is now moving ahead with debate, amendments and ultimately a final vote in the coming days on legislation that would have a profound impact on the American health care system — roughly one-sixth of the American economy. But it is entirely possible that by week’s end, they have passed nothing.
“Now we move forward towards truly great health care for the American people,” Mr. Trump said from the White House Rose Garden, where he was holding a news conference with the visiting prime minister of Lebanon. “This was a big step.”
Only two Republicans, Susan Collins of Maine and Lisa Murkowski of Alaska, voted against the procedural motion, though at least several other Republicans had been seen as possible holdouts. No Democrats voted in favor of the motion.
The debate to come will have broad implications for health care and households in every state.
Before senators cast their votes, protesters in the Senate gallery chanted, “Kill the bill, don’t kill us!” and “Shame, shame, shame!”
While the Senate was voting and before Mr. McCain showed up on the Senate floor, the majority leader, Mitch McConnell of Kentucky, engaged in a prolonged and intense conversation with Senator Ron Johnson, Republican of Wisconsin. Mr. Johnson had been an early critic of Mr. McConnell’s repeal bill, and on Tuesday, he held back his vote for an excruciatingly long time.
As soon as Mr. McCain arrived and voted aye, Mr. Johnson cast a yes vote.
Despite his vote to move ahead, Mr. McCain offered harsh words for the secretive process by which Senate Republican leaders came up with their bill to repeal and replace the health measure, and he delivered a pessimistic take on its chances.
“Asking us to swallow our doubts and force it past a unified opposition, I don’t think that’s going to work in the end — and probably shouldn’t,” Mr. McCain said, adding that it “seems likely” that the current repeal effort would end in failure.
Arizona is one of the 31 states that expanded Medicaid under the Affordable Care Act, and Mr. McCain’s remarks could be an ominous sign for other senators from states that expanded Medicaid, including the junior Republican senator from his state, Jeff Flake.
“We are ground zero for the failure of the exchanges, but we are also an expansion state,” Mr. Flake said. “I think all of us are concerned that we don’t pull the rug out from people.”
Just before the Senate vote, the Democratic leader, Chuck Schumer of New York, made an impassioned plea to Republicans.
“We know that A.C.A. is not perfect,” Mr. Schumer said. “But we also know what you’ve proposed is much worse. We can work together to improve health care in this country. Turn back now before it’s too late and millions and millions and millions of Americans are hurt so badly in ways from which they will never, ever recover.”
Given the divisions within their caucus, Senate Republican leaders were considering a fresh approach to keeping their repeal quest alive: They could try to reach agreement on a slimmed-down bill that would repeal a few major provisions of the Affordable Care Act, like the penalties imposed on people who go without insurance and businesses that do not offer insurance to their employees. Republicans leaders would not intend for such a bill to become law, but believe that it could win approval in the Senate.
That “skinny” bill could then be a basis for negotiations with the House.
Senate Republican leaders have struggled all year to fulfill their promise of repealing the 2010 health care law. By a vote of 217 to 213, the House approved a repeal bill in early May, but only after Republicans overcame their own difficulties in that chamber.
Mr. Trump kept up pressure on the Senate on Tuesday with Twitter posts. After the vote, he applauded the Senate, but was cutting toward Ms. Collins and Ms. Murkowski: “We had two Republicans that went against us, which is very sad, I think. It’s very, very sad for them.”
The successful procedural vote was also a moment of redemption, at least temporarily, for Mr. McConnell, who just last week appeared to have failed in his effort to put together a health bill that could squeak through the narrowly divided Senate.
That said, it remained far from certain whether Republicans would be able to agree on a bill in the days to come — and what exactly the contents of that bill would be. Mr. McConnell promised an “open amendment process” in which members of both parties could propose changes.
“This is just the beginning,” Mr. McConnell said. “We’re not out here to spike the football.”
For weeks, Mr. McConnell has been promoting and revising a comprehensive bill that would repeal the health law while also replacing it, but he has struggled to nail down the necessary support to pass that measure. Now that voting has begun, the most complete version of that replacement bill has yet to be assessed by the nonpartisan Congressional Budget Office, and without that assessment, the measure will need 60 Senate votes, a threshold it cannot reach.
An alternative would be to pass a narrower bill that would repeal the health law without putting in place a replacement, but even more Republicans have objected to that approach.
That proposal resembles a bill passed by the Senate in 2015 and vetoed by Mr. Obama in early 2016. But it would increase the number of people who are uninsured by 32 million in 2026, the budget office said.
Senator Rob Portman, Republican of Ohio, had anguished for weeks over provisions of Mr. McConnell’s repeal bill that would make deep cuts in projected Medicaid spending and roll back the expansion of the program under the Affordable Care Act.
Mr. Portman voted to move ahead with the debate on Tuesday after receiving a commitment from Mr. McConnell to allow a vote on a plan to provide financial assistance to people moving from an expanded state Medicaid program to private health insurance.
States could use the money, totaling $100 billion from 2019 to 2026, to help low-income people pay deductibles and other out-of-pocket costs when they receive medical care.
Mr. Portman worked on the plan with the Trump administration and with several other Republican senators from several states that have expanded Medicaid, including Shelley Moore Capito of West Virginia and Dean Heller of Nevada.
Democratic senators, consumer advocates and health care providers say the extra money is not nearly enough to make up for the cuts to Medicaid under the Republican bill.
Mr. Heller voted Tuesday to open the debate, but he made no commitment to vote for the repeal bill itself.
“If the final product isn’t improved for the state of Nevada, then I will not vote for it,” Mr. Heller said. “If it is improved, I will support it.”