Donald Trump wants to replace Obamacare. But it’s not that simple.

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 healthcare markets across the country and fuel an enormous political backlash as Trump was settling in for his first term. Many healthcare 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 and helped Sen. John McCain (R-Ariz.) develop a healthcare plan during his 2008 presidential campaign.

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.

Speaking at a recent healthcare forum at the U.S. Chamber of Commerce in Washington, the chamber’s senior lobbyist, Bruce Josten, implicitly chastised Republicans who have sought to capitalize on turmoil in some insurance markets this fall.

“It’s unfortunate that opponents of the ACA seem to be doing victory laps,” he said.

During the campaign, Trump provided little detail about how he would go about repealing and replacing the health law, though he said Thursday that healthcare would be a focus of his White House, along with immigration and jobs.

Congressional Republicans, though, have developed what could be a template for repeal.

Using Senate rules that exempt some budget-related legislation from filibuster, the GOP passed a bill last year that eliminated hundreds of billions of dollars provided by the health law to expand Medicaid coverage for very poor Americans and to subsidize health insurance for low- and moderate-income Americans on marketplaces created by the law.

The bill, which also scrapped the unpopular insurance mandate in the law that penalizes Americans who don’t have health insurance, envisioned a phase-out of the current law, giving Republicans time to develop an alternative.

President Obama vetoed the bill in January, but GOP leaders promised to revive it.

This summer, House Republicans also released a 37-page healthcare outline, which Speaker Paul Ryan (R-Wis.) advertised as a blueprint for potentially replacing the law.

The GOP plan would transform Medicaid, the government health program for the poor, by eliminating federal rules that establish who should be covered, such as poor children and pregnant women, and which benefits should be offered, leaving those decisions to states.

Medicaid and the related Children’s Health Insurance Program provide coverage to more than 70 million Americans.

Americans who don’t get coverage through an employer or through Medicare or Medicaid would qualify for a tax subsidy they could use to help offset the cost of a commercial insurance plan, similar to the system set up by the Affordable Care Act.

But Republicans argue these health plans would be more affordable than current plans available through Obamacare marketplaces because they would not be subject to as many federal regulations.

That outline should position Republicans to 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. “We will see alternatives presented and likely passed,” Troy predicted.

Whether this slower approach would satisfy the party’s base, which has been energized by more than six years of repeal calls, is unclear.

Also uncertain is whether voters would welcome the specifics of the GOP healthcare platform, which likely would mean less generous government assistance for millions of people.

For example, the House GOP plan would not link the tax credit to people’s incomes, as the current law does, potentially leaving lower-income consumers with less help to purchase insurance.

And independent analyses of previous GOP plans have noted that scaling back federal oversight of state Medicaid programs, while giving states more flexibility, could prompt some to roll back their healthcare safety nets.

The missing details of the House Republicans’ plan make it impossible to gauge how many people could lose or gain health coverage and how much more some Americans might have to pay for coverage.

Trump has generally embraced parts of the House GOP plan, including deregulating health insurance, restructuring Medicaid and changing the way federal government uses tax breaks to help people buy health coverage.

The president-elect has expressed less willingness to make major changes to Medicare, as congressional Republicans want. Ryan has long championed plans to replace the current government-run Medicare benefit with a system of vouchers that seniors could use to offset part of the cost of premiums for private health plans.

Trump has not indicated how much he will defer to Ryan and other congressional Republicans eager to push ahead with healthcare legislation. Trump and Ryan had a testy relationship during the campaign, and at one point, Trump suggested Ryan might not remain as speaker.

Twitter: @noamlevey

ALSO:

A doctor bikes across the country to ask Americans about Obamacare. This is how he ended up feeling hopeful

In Louisiana, the rush to sign up for Obamacare highlights a ‘long overdue’ demand for health insurance

Obamacare patients filled more prescriptions but paid less for drugs, study finds

UPDATES:

3 p.m.: This article was updated with comment from Trump.

11:40 a.m.: This article was updated with new Obamacare enrollment figures.

This article was originally published at 11:05 a.m.