On Capitol Hill, conservatives declared that they are not about to accept a health law replacement that remotely resembles the Affordable Care Act. And the I.R.S. adopted a policy for the coming tax season that could weaken the requirement for people to have insurance. The tax agency said it was reversing one aspect of an Obama administration plan after Mr. Trump, on his first day in office, issued an executive order instructing agencies to reduce burdens imposed by compliance with the Affordable Care Act wherever legally possible.

The proposed rules signal the Trump administration’s urgency in trying to keep other insurers from fleeing the market after Humana’s departure. The company said on Tuesday that it was losing money by insuring too many sick people without enough healthy ones enrolling. Humana had already scaled back its participation to 11 states this year, from 19 in 2016.

Mark T. Bertolini, the chief executive of Aetna, said Wednesday that the marketplaces were in “a death spiral,” and at a conference sponsored by The Wall Street Journal, he declined to say if his company would participate next year. And Molina Healthcare, one of the few insurers that seemed to be financially successful under the health law, reported on Wednesday that it was losing money in the marketplaces. It threatened to drop out if its concerns about a risk-sharing program requiring the company to pay hundreds of millions of dollars to other insurers were not addressed.

Dr. Patrick H. Conway, the acting administrator of the Centers for Medicare and Medicaid Services, said the rules proposed on Wednesday “will help protect Americans enrolled in the individual and small group health insurance markets while future reforms are being debated.”

But Democrats and consumer groups denounced the proposed rules, saying they would make it more difficult for people to enroll and increase costs for some consumers.