The collapse highlighted a harsh reality for Senate Republicans: While they freely assailed the health law when Mr. Obama occupied the White House, they could not come up with a workable plan to unwind it that would keep both moderate Republicans and conservatives on board. It was an enormous embarrassment for a party that rode electoral waves to control first the House, then the Senate and then the White House, but has not been able to deliver a major legislative victory.
âThis has been a very, very challenging experience for all of us,â Mr. McConnell said. âEverybodyâs given it their best shot, and as of today, we just simply do not have 50 senators who can agree on what ought to replace the existing law.â
The reaction on Wall Street was muted. Stocks spent most of the day lower as shares of health insurers declined, and the dollar, which has steadily lost ground for most of the year, slipped further.
Mr. Trump has considerable leverage to gum up the works of the Affordable Care Act. He could throw insurance markets into a tailspin at any time by cutting off the subsidy payments to insurers, as he has threatened to do. He could further destabilize the markets by not enforcing the mandate that most Americans have health insurance.
And he could cancel advertising and other efforts to encourage enrollment under the Affordable Care Act when the annual sign-up period begins in November. A barrage of negative statements from the administration could project an official view that the health law is collapsing, creating a self-fulfilling prophecy.
The lack of certainty over the subsidy payments, which go toward reducing out-of-pocket costs for low-income people, has been a major concern for insurers. The companies say premiums will be significantly higher without the funding, and some companies that have submitted rates to sell insurance in the market next year could decide to pull out.
âWith open enrollment for 2018 only three months away, our members and all Americans need the certainty and security of knowing coverage will be available and affordable for them,â said Justine Handelman, a senior executive at the Blue Cross Blue Shield Association.
While Mr. Trump has promised destruction, other Republicans signaled that they wanted to take a more constructive approach. Senator Lamar Alexander of Tennessee, chairman of the Senate health committee, announced that he would hold hearings in the next few weeks on stabilizing the individual health insurance market.
Members of both parties have ideas about how to stabilize insurance markets and hold down premiums. One possible action is to provide money for the payments to insurers for reducing customersâ out-of-pocket costs. In addition, two Democratic senators, Thomas R. Carper of Delaware and Tim Kaine of Virginia, want the federal government to help pay the largest claims through a backstop known as reinsurance. Senators of both parties also want to help people in counties where no insurer chooses to offer health plans through the Affordable Care Act marketplace â a real possibility in some places next year.
The Senate Democratic leader, Chuck Schumer of New York, implored Republicans to defy Mr. Trump and work with Democrats to strengthen insurance markets.
âThereâs a fork in the road for our Republican colleagues,â he said in an interview. âThey can do what Donald Trump said, which is sabotage the system out of anger and out of pique,â or they can work with Democrats on improvements to the health law.
âWhether they can resist Trump, I donât know,â Mr. Schumer said.
On Capitol Hill, Republicans and Democrats alike were trying to make sense of the repeal effortâs apparent downfall â and figure out what comes next.
The beginning of the end was on Monday night, when two Republican senators, Mike Lee of Utah and Jerry Moran of Kansas, came out in opposition to the latest version of Mr. McConnellâs bill to repeal and replace the health law. That left Republican leaders at least two votes short of what they needed to start debate.
Two other Republican senators, Ms. Collins and Rand Paul of Kentucky, had objected last week.
Mr. McConnell responded by outlining plans for a vote on a measure like the one vetoed by Mr. Obama in January 2016, which Mr. McConnell said would consist of a ârepeal of Obamacare combined with a stable, two-year transition period.â
Republican leaders had originally intended to proceed with a similar ârepeal and delayâ strategy after Mr. Trump won the presidency. But in January, Mr. Trump made clear he wanted a simultaneous repeal and replacement of the law, and congressional Republicans decided to follow that path.
A repeal-only route would have been disruptive. The Congressional Budget Office said in January that enacting the vetoed bill would increase the number of uninsured Americans by 18 million in the first year and 32 million by 2026, compared with current law. Premiums, it said, would increase 20 to 25 percent in the first year and double by 2026.
That bill would have eliminated the Affordable Care Actâs expansion of Medicaid and subsidies for the purchase of private insurance. But it would have left in place the lawâs requirement that insurers provide specific benefits, and the prohibition on denying coverage or charging higher premiums because of a personâs pre-existing medical conditions.
The repeal-only idea quickly ran into a wall on Tuesday.
âI did not come to Washington to hurt people,â Ms. Capito said in a statement, taking issue with both Mr. McConnellâs bill and the idea of repealing the health law âwithout a replacement plan that addresses my concerns and the needs of West Virginians.â
Ms. Murkowski said, âThereâs enough chaos and uncertainty already, and this would just contribute to it.â
The idea of repealing the health law without providing a replacement also spooked a bipartisan group of 11 governors, including Brian Sandoval of Nevada, an influential Republican critic of Mr. McConnellâs bill.
âThe Senate should immediately reject efforts to ârepealâ the current system and replace sometime later,â said the group, which included five Republicans, five Democrats and one independent. âThis could leave millions of Americans without coverage. The best next step is for both parties to come together and do what we can all agree on: fix our unstable insurance markets.â
Mr. McConnell appears determined to drive the effort to a final public showdown with a procedural vote that would let the Senate consider the repeal-only measure. He can afford to lose only two Republican senators, with Vice President Mike Pence breaking the tie. But he already appears to have lost at least three: Ms. Collins, Ms. Murkowski and Ms. Capito.
More could still defect, unwilling to be recorded voting for a procedural step in what is all but certain to be a doomed exercise. But that could be an awkward stance for Republicans who voted for the repeal bill that the Senate passed in 2015 and Mr. Obama vetoed.
âIf youâre not willing to vote the same way you voted in 2015,â Mr. Paul said on Tuesday, âthen you need to go back home, and you need to explain to Republicans why youâre no longer for repealing Obamacare.â