(Bill Oxford/Getty Images/iStock)

For the past six years, no law has served as a larger GOP whipping post than the Affordable Care Act, and the Republican sweep Tuesday of political Washington has imperiled the ACA’s expansive reach, putting at risk the insurance that more than 20 million Americans have gained.

During the final week of his campaign, President-elect Donald Trump vowed to repeal the 2010 health-care law so swiftly that he might summon Congress into a special session to accomplish the task. “We will do it, and we will do it very, very quickly. It is a catastrophe,” he said.

On Wednesday, House Speaker Paul D. Ryan (Wis.) and other congressional Republicans voiced fresh determination to complete the deed. After dozens of fruitless repeal votes in the House, and a major rescission attempt this year that President Obama blocked, Ryan noted that “now we have President Trump coming, who is asking us to do this.”

According to lawmakers and health-policy analysts, the GOP majorities in both chambers are likely to employ Congress’s reconciliation process to reverse critical aspects of the statute that involve federal spending, such as the subsidies helping millions of working- and middle-class Americans afford health plans. But analysts said a political path is less clear to dismantling other parts of the law, such as its insurance marketplaces, or to instituting a set of conservative health-care approaches.

Donald Trump has campaigned to repeal and replace the Affordable Care Act, otherwise known as Obamacare, once he gets into office. Now that he's won the presidency with a majority Republican House and Senate, that feat might not prove to be too easy. Wonkblog's Max Ehrenfreund explains. (Daron Taylor/The Washington Post)

The ACA’s most ardent supporters have begun a counteroffensive to stoke opposition to reversing ways in which the law has upgraded coverage and made it more affordable for some consumers. Families USA, a liberal consumer-health lobby, convened an afternoon call with more than 1,000 people from all 50 states to begin mapping a grass-roots campaign.

“The clock is ticking, because Republicans appear to be saying health care is going to be the first item on their list with repeal of the ACA being the banner for that,” said Ron Pollack, Families USA’s executive director for three decades. “This will be the most intense fight I remember. . . . One should never underestimate an extraordinary backlash that occurs when people have something that they really value and it is taken away.”

The Affordable Care Act, enacted in the spring of 2010 with virtually no GOP support, is a 2,000-page statute that has ushered in the broadest changes to the health-care system in ­half a century. With Trump’s election, “the ACA as we know it would seem to be toast,” said Larry Levitt, a senior vice president at the Kaiser Family Foundation.

“I don’t think there has been a reversal of any public benefit that would be as large as this,” Levitt said. The only other significant reversal by Congress of a major health-care policy — the expansion of Medicare to include catastrophic coverage — took place in 1989 before the benefit took effect.

The challenge for Republicans, Levitt added, “comes now in trying to come to some consensus about how to unwind it and what to replace it with.”

Senate Majority Leader Mitch McConnell (Ky.), one of the ACA’s loudest critics, declined Wednesday to spell out details of the procedures Republicans would use to abolish the law. But in a media briefing to celebrate the Senate preserving its majority, he said, “All of that is underway.”

These former Obama strongholds sealed the election for Trump

Congress demonstrated in January that it could use the Senate’s reconciliation process — requiring just 50 votes — to send a repeal bill to the White House. Although GOP lawmakers understood that Obama would veto that legislation, “it was a strategic move,” said Tevi Troy, an ACA critic who is a former deputy health secretary and the chief executive of the American Health Policy Institute. “Congress intentionally set it up so they could demonstrate a legislative pathway.”

Under that bill, there would have been a two-year transition before any features of the law ended. The idea was to give congressional Republicans breathing space to design a replacement and pass a new law.

But with the GOP holding 51 seats in the next Senate, it is unclear whether it will be able to amass the 60 votes needed to overcome a potential filibuster and adopt an alternative to the ACA.

If the GOP repeals the law but then is “unable to agree in two years on what will take its place, that is the nightmare scenario,” said John McDonough, a professor at the Harvard School of Public Health who worked intensively on the ACA’s creation as an aide to the late senator Edward M. Kennedy (D-Mass.).

Still, the reconciliation process could have sweeping effects if it reversed both the insurance subsidies in ACA marketplaces and a set of taxes that help pay for other parts of the law. If Congress undid a Medicare payroll tax, McDonough said, it would “actually implement one of the largest tax cuts on high-income families ever.”

Trump has said that he favors keeping one key aspect, which outlaws the old practice by many insurers of refusing to cover people with preexisting medical problems or charging them more than other customers. The insurance industry has long said it would have a hard time abiding by this rule unless virtually all Americans are required to have insurance — a central feature of the ACA that Trump wants to cut.

Trump’s campaign never put details on a set of conservative health-care ideas that he sketched out as a candidate. They included converting Medicaid from an entitlement program for lower-income Americans to annual block grants to states. Also proposed was letting individuals deduct the cost of insurance from their taxes.

Even without details, congressional budget analysts and outside health-policy experts have estimated the likely impact of dismantling the ACA and replacing it with Trump’s health policies.

The Congressional Budget Office forecast that, over the coming decade, repealing the law would cause the deficit to grow by $353 billion, while the number of people with insurance would fall by about 24 million. The Rand Corp. has predicted that in 2018, the first full year of Trump’s tenure, his campaign’s health plan would add nearly $6 billion to the deficit, primarily by undoing a slowdown in Medicare payments under the law. It also would decrease the number of insured by about 20 million people, according to Rand.

In the short term, the sudden doubt about the law’s future also has the potential to confuse — and perhaps thwart — the fourth open-enrollment season for ACA health plans through HealthCare.gov and similar state-run insurance marketplaces. The three-month sign-up period began Nov. 1 amid surging insurance premiums and diminished coverage options in many parts of the country.

In his first briefing after the election, White House press secretary Josh Earnest said there were no specific steps the president or his aides plan to take to shore up policies with which his successor disagrees. “This administration is going to continue to make a strong case that people should go to HealthCare.gov, consider the options that are available to them and sign up for health care,” Earnest said.

Paul Kane contributed to this report.