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President Joe Biden has pushed an expansion of the program, signing into law billions of dollars in that expanded who qualified for the health insurance and lowered its cost. Millions of additional Americans can now pay monthly premiums of just a few dollars to get coverage.

The increased enrollment is “no coincidence," Biden said in a statement. “When I took office, I made a promise to the American people that I would bring down the cost of health care and prescription drugs, make signing up for coverage easier, and strengthen the Affordable Care Act, Medicare, and Medicaid."

But incoming President-elect Donald Trump has maligned “Obamacare� for years. He unsuccessfully tried to dismantle it during his first term, and has promised changes — without offering a concrete plan — during his second term. Enrollment dropped during Trump's first term, with his administration investing less money in the program, including for navigators who help people enroll in the coverage.

And the tax credits that made the health care coverage more affordable for millions will expire at the end of this year, unless Congress passes a new law.

Lawmakers will face a pressure campaign from hospitals and insurance companies, which supply coverage plans for the marketplace, to continue the tax credits. A newly formed coalition of the biggest and most powerful health care entities — including the nation's top health insurers, largest health care systems and notable medical associations — has launched a campaign called “Keep Americans Covered� to lobby Congress on the issue.

Still, it's an uphill fight with Republicans, some of whom voted against the Affordable Care Act initially and others who voted to repeal it years later. Still, some might pause at the idea of effectively stripping their constituents of health care coverage.

Republican Sen. Lisa Murkowski that Congress would “need to continue these premium tax credits," in an interview about rising health care costs.

But other Republicans have , especially with continued growth in enrollment. Last year, a group of GOP representatives called for an investigation into Affordable Care Act signups, citing concerns that people are defrauding taxpayers by reporting inaccurate income levels in order to qualify for cheaper health care coverage.

In a call with reporters Tuesday, Biden administration officials pushed back on that, saying that automated systems verify a person's income against the previous year's tax filings.

Trump, meanwhile, has described the Affordable Care Act as “costly,� and says the health care coverage it offers is “lousy.�

But he has still not offered a full plan for how he would make it better.

“We have concepts of a plan that would be better,� he said during an interview on NBC's "Meet the Press" last month.

Open enrollment on ends on Jan. 15.