WASHINGTON — Republicans, who for six years have promised to repeal and replace the Affordable Care Act, will finally get their chance to do it. But even with control of the White House and Congress, it’s unclear whether the GOP can pull it off.

While rolling back ObamaCare, as President-elect Donald Trump has promised to do in his first days in office, could be accomplished relatively easily, enacting the complex legislation necessary to replace the law while protecting millions of Americans who depend on it for coverage may prove daunting.

It also would represent an unparalleled effort by a new president to dismantle a major government program and replace it with something new.

“It’s a very big challenge,” said James C. Capretta, a leading conservative health policy expert and former Bush administration official now at the American Enterprise Institute.

“There will be a lot of pressure to do something and to do it relatively quickly. But the big complication with repeal and replace has always been, how do they handle the replace part?”

Indeed, congressional Republicans have never advanced replacement bills through an arduous committee process, submitted the legislation to rigorous budgetary analysis or contended with the potential impact on constituents who could see their out-of-pocket medical costs soar.

GOP lawmakers have held more than 50 repeal votes over the past six years. While most of those were symbolic, the next one could strip insurance coverage from more than 20 million Americans, according to independent estimates.

Such a step to roll back government benefits — unprecedented in modern U.S. history — likely would roil state health care markets across the country and fuel an enormous political backlash as Trump was settling in for his first term. Many health care industry stocks fell dramatically Wednesday amid investor worries about a potential new repeal push.

“You can’t just pull the plug on 20 million people,” warned Gail Wilensky, who headed the Medicare and Medicaid programs under President George H.W. Bush.

Underscoring the challenge, more than 100,000 people signed up for health coverage through the law on Wednesday, the day after Trump’s election.

Within hours of Trump’s victory, several activist groups already were threatening to mobilize to defend the ObamaCare coverage expansion.

“We at Families USA are going to be on a total war footing,” said Ron Pollack, the group’s executive director and a leading consumer advocate. “We will fight at the grass-roots level and in the halls of Congress.”

And hospital and physician groups, health insurers, consumer advocates and even some conservative business leaders already were urging elected officials to end the protracted political battle over ObamaCare and take constructive steps to make the law work better.

Republicans could develop replacement legislation over the next year, said Tevi Troy, a former health official in the George W. Bush administration who now heads the American Health Policy Institute.

Troy said there may not be a single replacement bill, but rather a series of smaller bills over the next two years that address various challenges.