WASHINGTON — President Trump on Wednesday backed away from his endorsement of a bipartisan Senate proposal to stabilize health insurance markets, throwing the legislative effort into doubt even as the chief architect of the deal predicted that it would become law before the end of the year.

The latest actions by the White House confused Republicans on Capitol Hill and irked Democrats — but in the end, their effect was not clear. The effort to calm roiled insurance markets appears destined for a showdown in December, when supporters of the compromise, drafted by Senators Lamar Alexander, Republican of Tennessee, and Patty Murray, Democrat of Washington, will have the most leverage.

The mixed signals began Tuesday, when Mr. Trump appeared to give his blessing to the deal to restore subsidies to health insurers that he said days before he would cut off. Mr. Alexander and Ms. Murray agreed on legislation that would continue federal payments to insurance companies through 2019 to reimburse them for discounts that they are required to provide to millions of low-income people who have coverage under the Affordable Care Act, popularly known as Obamacare.