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, âKill the bill, donât kill us!â and âShame, shame, shame!â
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. McConnellâs 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.
âAsking us to swallow our doubts and force it past a unified opposition â I donât think thatâs going to work in the end, and probably shouldnât,â Mr. McCain said, adding that it âseems likelyâ that the current repeal effort will end in failure.
Arizona is one of the 31 states that expanded Medicaid under the Affordable Care Act, and Mr. McCainâs remarks could be an ominous sign for other senators from states that expanded Medicaid, including the junior Republican senator from his state, Jeff Flake.
âWe are ground zero for the failure of the exchanges, but we are also an expansion state,â Mr. Flake said. âI think all of us are concerned that we donât pull the rug out from people.â
Just before the Senate vote, the Democratic leader, Chuck Schumer of New York, made an impassioned plea to Republicans.
âWe know that A.C.A. is not perfect,â Mr. Schumer said. âBut we also know what youâve proposed is much worse. We can work together to improve health care in this country. Turn back now before itâs too late and millions and millions and millions of Americans are hurt so badly in ways from which they will never, ever recover.â
Given the divisions within their caucus, Senate Republican leaders were considering a new approach to keeping their repeal quest alive: They could try to reach a deal 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 they believe that it could win approval in the Senate.
That âskinnyâ bill could then be a basis for negotiations with the House.
Republican leaders in Congress 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: âWe had two Republicans that went against us, which is very sad, I think. Itâs very, very sad for them.â
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 â and what exactly the contents of that bill would be. Mr. McConnell promised an âopen amendment processâ in which members of both parties could propose changes.
âThis is just the beginning,â Mr. McConnell said. âWeâre not out here to spike the football.â
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 that approach is not expected to have enough support to pass, either.
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. McConnellâs 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, 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.
âIf the final product isnât improved for the state of Nevada, then I will not vote for it,â Mr. Heller said. âIf it is improved, I will support it.â