President-elect Donald J. Trump never revealed the “concepts” of the plan he said he had to replace the Affordable Care Act, the law that he tried unsuccessfully to kill during his first term.

But the fact that Republicans have gained control of the Senate — and possibly the House — could give Mr. Trump an opening to try and transform the 2010 health law and remake the nation’s health care system.

Key to that strategy, health policy experts said, is simple inaction. Major subsidies that lawmakers approved during President Biden’s term that have lowered the cost of plans are set to expire next year. Republicans could allow them to sunset, a move that could deprive roughly 20 million Americans of extra financial help for coverage on the Affordable Care Act’s marketplaces.

The subsidies, which are estimated to cost more than $300 billion if extended for a decade, helped Obamacare enrollment almost double during President Biden’s term, shattering records. The Congressional Budget Office estimates that 3.4 million people will lose insurance if the subsidies expire and the cost of plans rises. Lower-income Americans would still receive some federal assistance, while higher-income people would lose it altogether.

“Because premium payments would go up for so many people, you’re likely to see a lot of people dropping coverage,” said Cynthia Cox, an Obamacare expert at KFF, a nonprofit health policy research group. And since the marketplaces are a last resort for many Americans, she said, many of those with plans now would likely be uninsured.

The future of the law has taken on new urgency, not just because of Mr. Trump’s record in office seeking to reverse it. Speaker Mike Johnson, who would likely continue in his role in a Republican majority, said last week at a campaign stop that his caucus would seek “massive reform” if Mr. Trump was elected again, with health care a top priority in the early months.

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