But the victory could be fleeting: Senate Republicans still have no agreement on a repeal bill that they can ultimately pass to uproot the law that has provided health insurance to millions of Americans.
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 effect on the American health care system — roughly one-sixth of the United States’ economy. But it is entirely possible that by week’s end, the senators will 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 Tuesday night vote was on a comprehensive amendment that included disparate proposals calculated to appeal to conservatives and moderates in the Republican caucus.
One proposal, offered by Senator Ted Cruz, Republican of Texas, would have allowed insurers to sell stripped-down health plans, without maternity care or other benefits required by the Affordable Care Act, if they also sold plans that included such benefits.
“You shouldn’t have to buy what the federal government mandates you must buy,” Mr. Cruz said. “You should choose what meets the needs for you and your family.”
The amendment also included money to help pay out-of-pocket medical costs for low-income people, including those who buy private insurance after losing Medicaid coverage as a result of the Senate bill. This proposal was devised by Senator Rob Portman, Republican of Ohio, and other senators from states that have expanded Medicaid under the Affordable Care Act.
But nine Republicans, spanning the party’s ideological spectrum, voted against the package.
The debate to come will have broad implications for health care and households in every state, and emotions are high.
Before senators voted to start the debate in midafternoon, protesters in the Senate gallery chanted, “Kill the bill, don’t kill us!” and “Shame, shame, shame!”
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 law, 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. Still, Mr. McCain voted with Republican leaders in favor of the comprehensive replacement plan on Tuesday night.
Arizona is one of the 31 states that expanded Medicaid under the Affordable Care Act, and Mr. McCain’s remarks could reflect concerns of 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 new 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. Republican leaders would not intend such a bill to become law, but they believe that it could win approval in the Senate.
That “skinny” 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 procedural 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 Senator Mitch McConnell of Kentucky, the majority leader, 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 support needed to pass that measure. The nonpartisan Congressional Budget Office has yet to assess the most complete version of that legislation, which includes the proposals by Mr. Cruz and Mr. Portman.
Without that assessment, the measure needed 60 Senate votes, and it failed that test on Tuesday night.
The Senate is also expected to vote on a measure that would repeal the health law without putting in place any replacement, but that approach does not appear 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.
Mr. Portman 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 being assured that the Senate would vote on his plan to provide financial assistance to people moving from an expanded state Medicaid program to private health insurance.
States could have used 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 states that have expanded Medicaid, including Shelley Moore Capito of West Virginia and Dean Heller of Nevada.
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.”