President Trump has begun a fresh assault on the Affordable Care Act, declaring his intent to come up with a new health-care plan and backing a state-led lawsuit to eliminate the entire law.

But Trump and Republicans face a major problem: The 2010 law known as Obamacare has become more popular and enmeshed in the country’s health-care system over time. Thirty-six states and the District of Columbia have expanded Medicaid — including more than a dozen run by Republicans — and 25 million more Americans are insured, with millions more enjoying coverage that is more comprehensive because of the law.

Even Republicans who furiously fought the creation of the law and won elections with the mantra of repeal and replace speak favorably of President Barack Obama’s signature domestic achievement.

“Quite obviously, more people have health insurance than would otherwise have it, so you got to look at it as positive,” Sen. Charles E. Grassley (R-Iowa) said in a recent interview.

Ten years ago, Grassley was at the forefront of GOP opposition to the law, ominously pushing the debunked claim that it would allow the government to “pull the plug on grandma” by creating “death panels.”

Today, Grassley is chairman of the Senate Finance Committee, the panel that would be responsible for drafting a new health-care law, and he has shown little enthusiasm for Trump’s call for congressional Republicans to produce a replacement for the ACA.

Republicans from states that embraced the law’s Medicaid expansion also concede that it has benefited large portions of the low-income population, many of whom were previously uninsured.

After a district judge ruled that the Affordable Care Act was unconstitutional, the Justice Department said the whole law should be invalidated. (Jenny Starrs/The Washington Post)

“For the people who are in that tranche of expanded Medicaid, I think it has been very helpful,” said Sen. Shelley Moore Capito (R-W.Va.). Nearly one-third of West Virginians are on Medicaid, and the percentage of uninsured has dropped by about 56 percent since 2013.

It is an astonishing turn in the circumstances of a polarizing law that the House GOP voted more than 60 times over nearly a decade to scrap and almost scuttled in 2017 — and one that Trump remains intent on destroying.

In the past week, the Justice Department sought to expedite the legal challenge to the law, asking the U.S. Court of Appeals for the 5th Circuit to hold oral arguments in the case in July. The lawsuit, spearheaded by conservative states and embraced by Trump’s Justice Department, would destroy the ACA, upending coverage for 12 million people newly eligible for Medicaid and 9.2 million more who receive federally subsidized coverage via the law’s state-based marketplaces.

The lawsuit also would wipe out consumer protections established by the law, such as allowing children to remain on their parents’ health-care plans up to age 26 and requiring insurers to accept those with preexisting medical conditions without charging them more.

Kathleen Sebelius, who was secretary of the Department of Health and Human Services during the Obama administration, said that in the first few years after the law’s implementation, “there was not tangible evidence of what this was going to look like.”

By 2015, however, Sebelius said, the law’s coverage provisions were firmly in place.

“By that time, you really began to see practical benefits,” she said in an interview with The Washington Post. “Then folks are in a different situation, where they are now saying, ‘It might not be perfect, but I do not want to lose my health care.’ ”

Passage of Obamacare has most benefited Americans who lacked coverage before its enactment — about 50 million at the time — and people with workplace-sponsored coverage, whose plans must cover benefits more generously. Insurers in the individual and group markets cover preventive services without charging co-pays and are prohibited from placing annual or lifetime caps on coverage.

The law also sought to save money for seniors by filling in Medicare’s drug coverage gap.

The public’s increasing reliance on the ACA was reflected in the dramatic failure of congressional Republicans to roll back the law or even unify around a plan to replace it as it has grown in popularity.

In March, 50 percent of Americans had a favorable opinion of the ACA, while 39 percent viewed it unfavorably, according to the Kaiser Family Foundation. That is near the record low of unfavorable views of the health reform law — 37 percent viewed it unfavorably in February.

Soon after Trump was elected, favorable opinion of the ACA grew.

Partisan differences still abound: 8 in 10 Democrats viewed the ACA favorably in March, while almost as many Republicans (75 percent) viewed it unfavorably. Independents were split, 45 percent favorable, 41 percent unfavorable.

Politically, Republicans used their strong opposition to the law to win the majority in the House in 2010 and the Senate in 2014 — just over a year after the disastrous launch of the government website on Oct. 1, 2013.

The reverse was true in last year’s midterm elections, as Democrats effectively used the GOP desire to eliminate the law — and its protections for people with preexisting medical conditions — to defeat Republicans. Democrats flipped at least 40 seats en route to capturing the House majority.

Bowing to pressure from some in his own party, Trump recently backed off a new pledge to take another crack at eliminating the ACA and said a vote on a GOP health plan — still unformed — would be delayed until after the 2020 election.

But the administration has also tried to peel back the ACA in smaller ways, expanding leaner plans that do not comply with all of its coverage requirements, conducting only limited marketplace outreach and cutting off extra subsidies that help the lowest-
income enrollees with out-of-pocket costs — a move that ironically resulted in making the premium subsidies more generous.

Despite the attacks, the ACA has become increasingly entrenched in the American health-care system.

“I don’t think most people understand the details, but they have a real sense that it would be a setback for health-care coverage in this country if the ACA were wiped out,” said former congressman Henry Waxman (D-
Calif.), who helped write the law.

If the law were to be eliminated “it would be total chaos,” said former Ohio governor John Kasich, one of the leading Republicans to embrace parts of the ACA.

“It provided a lot of coverage to a lot of people,” he added.

In the 2018 midterms, voters in the Republican-leaning states of Nebraska, Utah and Idaho approved ballot initiatives expanding Medicaid. The Centers for Medicare and Medicaid Services recently approved Maine’s Medicaid expansion, which was also approved via the ballot.

Even in states that have tried to reject the ACA wherever possible, the law has had a marked impact. Florida, Texas, North Carolina and Georgia — which are among the 14 states that still refuse to expand Medicaid — have the most marketplace enrollees, according to the Kaiser Family Foundation.

While most legal scholars think it is unlikely the Supreme Court will strike down the ACA should it ultimately hear the case, the lawsuit and the Trump administration’s response frustrates many state leaders who have watched their uninsured rates decline as the ACA has become enmeshed in their systems.

Two Republican state attorneys general — Dave Yost in Ohio and Tim Fox in Montana — recently filed a legal brief outlining the consequences for their states should the courts strike down the ACA.

“The court’s decision, if affirmed, will deprive millions of non-elderly Ohioans and Montanans of coverage for preexisting conditions,” the pair wrote. “It will also negatively affect countless others who organized their affairs in reliance on the Act’s many unrelated provisions.”

Health and Human Services Secretary Alex Azar has stressed that the ACA is the law of the land despite the ongoing lawsuit. “A legal interpretation of a court case is not a policy position about what we want to have happen for people with preexisting conditions,” Azar told Congress this month.

Democrats have often acknowledged that the ACA is not a perfect law and can be improved, particularly by making monthly premiums more affordable. But bipartisanship remains elusive as Trump and many in the GOP demand that the law be scrapped.

“There are problems with the law, and we should be working together to resolve those matters,” Waxman said. “Instead, we’re still, 10 years out, fighting over whether the law should be in effect and should be on the books at all.”

Erica Werner and Emily Guskin contributed to this report.