But with at least three House Democrats opposing the toughest version of the measure, and at least one Senate Democrat, Kyrsten Sinema of Arizona, against it, government negotiating power appears almost certain to be curtailed, if not jettisoned. The loss would be akin to Republicans’ failure under Mr. Trump to repeal the Affordable Care Act, after solemn pledges for eight years to dismantle the health law “root and branch.”

And after so many campaign-trail promises, Democrats could be left next year with a lot of explaining to do.

“It would mean that the pharmaceutical industry, which has 1,500 paid lobbyists, the pharmaceutical industry, which made $50 billion in profits last year, the pharmaceutical industry, which pays its executives huge compensation packages, and which is spending hundreds of millions of dollars to defeat this legislation, will have won,” Senator Bernie Sanders, the Vermont independent and Budget Committee chairman, said on Wednesday. “And I intend to not allow that to happen.”

It is not clear how Mr. Sanders can pull that off.