But Republicans in Congress are divided. Some worry that ending the subsidies would hurt their constituents. Others are loath to do anything that could be seen as propping up the health law that they had promised to tear down.
For their part, Democrats are convinced that any blame for rising premiums and shrinking choices will fall on Republicans, who now control the White House and Congress. After spending the year trying to preserve the Affordable Care Act, Democrats did not appear ready on Friday to make major concessions.
“Unless our Republican colleagues act, the American people will know exactly where to place the blame when their premiums shoot up and when millions lose coverage,” said Senator Chuck Schumer of New York, the Democratic leader.
The uncertainty over the payments to insurers has loomed over Congress for months, but Mr. Trump’s action is forcing lawmakers to confront the issue head on less than three weeks before the start of the annual open enrollment season under the Affordable Care Act.
Even with the president’s move, it was far from clear whether lawmakers could agree on steps to continue the payments. The negotiations between Mr. Alexander and Ms. Murray had stalled over how much latitude to give states seeking waivers from requirements of the Affordable Care Act.
Still, Ms. Murray expressed optimism and called on Republican leaders to support the efforts.
About seven million people benefit from the cost-sharing subsidies. The president’s decision, by destabilizing insurance markets and driving up premiums, could adversely affect millions of others who buy insurance on their own and do not receive federal subsidies, health policy experts said.
Representative Tom Reed, Republican of New York and co-chairman of a bipartisan group of lawmakers called the Problem Solvers Caucus, said Mr. Trump’s decision “increased the stakes” for Congress. More than two months ago, Mr. Reed’s group offered a series of proposals to shore up insurance markets, including funding the subsidies.
“It’s only going to get worse as this marketplace continues to destabilize,” Mr. Reed said. “If we stay where we are and do nothing, I think this is going to be a pox on all of our houses.”
Representative Mark Meadows, Republican of North Carolina and the chairman of the hard-line conservative House Freedom Caucus, said he had been in discussions with other Republicans over proposals to drive down premiums that could be tied to a continuation of the subsidies for insurers.
“The fundamental question is, what do you get in terms of relief for insurance premiums and health care costs, broadly, in exchange for continuing a bailout to the insurance companies in the short run or long run?” Mr. Meadows said.
As lawmakers mulled how to move forward, doctors, hospitals, insurers, state insurance commissioners and advocates for patients denounced Mr. Trump’s decision.
“Our patients will ultimately pay the price,” said Dr. David O. Barbe, the president of the American Medical Association. “We urge Congress to accelerate its efforts to reinstate these payments before further damage is done.”
Governors also weighed in, expressing alarm at the termination of the subsidies. “We are deeply concerned that the administration has declined to continue these payments, further increasing uncertainty for state marketplaces,” the National Governors Association said.
Gov. Brian Sandoval of Nevada, a Republican, told The Nevada Independent that the president’s decision was “going to hurt kids.” He continued: “It’s going to hurt families. It’s going to hurt individuals. It’s going to hurt people with mental health issues. It’s going to hurt veterans. It’s going to hurt everybody.”
The attorneys general of 18 states, including California, Massachusetts and New York, filed suit on Friday to prevent any interruption in the payments.
But the Trump administration insisted that a federal district judge was right in ruling last year that the payments were unconstitutional because Congress never explicitly provided money for them.
“Congress has the power of the purse, and it is up to Congress to decide which programs it will and will not fund,” Attorney General Jeff Sessions said in a legal opinion, adding: “The executive branch cannot unilaterally spend money that Congress has not appropriated.”
The cost-sharing subsidies are available to people with incomes of 100 percent to 250 percent of the federal poverty level, or about $12,060 to $30,150 a year for an individual.
“Cost sharing reductions are critical to low income Americans,” Representative Carlos Curbelo, Republican of Florida, wrote on Twitter. “Congress should guarantee their funding through the appropriations process.”
In fact, the consequences of the president’s action would ripple through the individual market. The premium increases would be felt most by middle-class consumers who do not qualify for federal aid with either their premiums or their out-of-pocket costs.
The government’s costs are also likely to rise, since subsidies to purchase insurance through the marketplaces increase as premiums rise. The Congressional Budget Office said in August that if the cost-sharing subsidies were cut off, premiums would shoot up 20 percent next year, and federal budget deficits would increase by $194 billion in the coming decade.
Yet conservative Republicans made clear they have no appetite for providing billions of dollars to insurers.
“Under no circumstance should Congress attempt to expand Obamacare by cutting a check for President Obama’s bailout of insurance companies,” said Representative Mark Walker of North Carolina, the chairman of the conservative Republican Study Committee.
Representative Dave Brat, a conservative Republican from Virginia, said Mr. Trump was simply correcting a “constitutional error” made by the Obama administration.
Mr. Brat said he did not want to provide money for the subsidies because they would just “prop up a failing system in which the government coerces people to buy gold-plated insurance policies that no one can afford.”
The showdown increased tensions over a fiscal deadline facing lawmakers. Mr. Trump and Democrats struck a deal in September that included a stopgap spending measure to keep the government funded through Dec. 8.
Now, the fate of the cost-sharing reduction payments could wind up intertwined in the negotiations over a spending measure to avert a government shutdown. Already, the contentious issue of immigration could become part of those spending talks, as Democrats are determined, in the coming months, to secure protections for the young undocumented immigrants known as Dreamers.
Mr. Schumer said the coming spending measure offered a “very good opportunity” to secure money for the subsidies.