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.”