(CNN) The extended battle over the Affordable Care Act will push deeper into uncharted waters Tuesday, when lawyers from a coalition of Republican-controlled states and the Trump administration argue for overturning the law in a federal court.

With this legal challenge -- and promises from President Donald Trump that he will try again to rescind the law if he's reelected in 2020 -- Republicans are prolonging the political struggle against the Affordable Care Act to an unprecedented extent.

These new threats to the ACA are coming more than nine years after then-President Barack Obama signed it into law in March 2010. None of the other pillars of the American social safety net -- Social Security, Medicare or Medicaid -- faced efforts to repeal them nearly so long after they were instituted.

"There just isn't a precedent for this kind of long-range battle," says Donald F. Kettl, a professor at the Lyndon B. Johnson School of Public Affairs at the University of Texas at Austin.

The political and policy stakes in this struggle are enormous. If the Trump administration and Republican-controlled states win their legal effort to completely invalidate the law, 20 million people would lose health insurance, and hospitals and other health care providers would be forced to provide $50 billion more in uncompensated medical care in just the first year, according to projections by the nonpartisan Urban Institute.

"It would be a devastatingly disruptive change to both health care providers and to consumers," says Linda Blumberg, an institute fellow at the Urban Institute.

The political consequences could be just as seismic. In 2018, Democratic promises to defend the ACA -- particularly its provisions guaranteeing coverage for patients with preexisting conditions -- were key to the gains that swept them back into control of the House of Representatives.

Now the Trump administration and an 18-state coalition led by Texas are appearing before the 5th US Circuit Court of Appeals on Tuesday to argue that those protections, and the rest of the ACA, should be invalidated. If the 5th Circuit agrees, the Supreme Court likely would be forced to decide the law's fate next June -- with the presidential campaign in full swing.

"It is Trump's nightmare, that at the height of the 2020 campaign he could be in the Supreme Court trying to overturn protections for people with preexisting conditions," says Democratic consultant Jesse Ferguson, in a verdict privately echoed by many GOP strategists. "I think people underestimate what this could all mean."

In addition to the administration's legal challenge, Trump has repeatedly declared that he will again try to legislatively repeal and replace the law if he's reelected and Republicans regain unified control of Congress in 2020. That means the GOP will almost certainly run in 2020 on repealing the ACA for the third consecutive presidential election, after Trump in 2016 and Mitt Romney in 2012 pledged to undo the law.

The Republican-led House voted repeatedly to repeal the law during Obama's final years in office, and in 2017 Trump and Republican leaders passed repeal through the House and came within one vote of rescinding the law in the Senate.

The latest challenge crystallized when a conservative federal district judge last December upheld the argument from the coalition of GOP states that the entire law should be dismissed after Congress effectively eliminated the individual mandate penalty by reducing it to $0 in the 2017 tax law. After initially taking a narrower position, Trump's Justice Department has now joined the states in arguing to the appellate court that the entire law should be thrown out. A coalition of 20 states led by California Attorney General Xavier Becerra is spearheading the legal effort to defend the law.

Medicare and Social Security faced shorter threats

Nothing like this prolonged trench warfare followed passage of the social safety net's other central strands.

After President Franklin D. Roosevelt signed Social Security into law in 1935, for instance, the 1936 Republican presidential nominee, Alf Landon, called the new law "a cruel hoax" and ran on repealing it. But after Landon won only two states, Republicans essentially conceded the fight. The next GOP nominee, Wendell Willkie, ran in 1940 on expanding Social Security. And while Republicans in Congress continued guerrilla efforts to retrench the law through the 1940s, the party never again seriously contemplated trying to eliminate it.

The fight over Medicare, the giant federal health care program for the elderly, fizzled even more quickly. Barry Goldwater, the staunchly conservative GOP presidential nominee in 1964, fiercely opposed Medicare. But once Congress created it -- with 13 Republicans in the Senate and 70 in the House voting yes -- and President Lyndon Johnson signed it into law in 1965, opposition collapsed almost immediately. Richard Nixon, the GOP presidential nominee in 1968, treated Medicare as settled law and did not propose to repeal it; indeed, no Republican presidential nominee ever ran on repealing Medicare.

Medicaid, the federal-state partnership to provide health care for the poor, was more controversial, but within seven years of its passage, every state except Alaska and Arizona had committed to participate -- and both of them joined later. Starting with Ronald Reagan's presidency, Republicans have repeatedly sought to transform the program from an entitlement that guarantees benefits to a block grant operating under a fixed budget. But they have not sought to fully repeal the program.

The ebbing of resistance to these other safety net programs reinforced the belief of politicians and political scientists alike that it is virtually impossible to revoke benefits once the government has provided them.

"The bottom line is it's very hard politically to give people something then to tell then later you're doing them a big favor by taking it away," says Kettl.

The potential impact of repealing the ACA is magnified because it has become woven into the expectations of not only patients, but also the vast medical industry of doctors, hospitals, drug companies and medical device manufacturers.

"There's not a place in the health care system that wouldn't be affected by this repeal, and that's why, in a lot of ways, it's very difficult to even imagine how you even implement the full repeal at this point," says Blumberg.

In two recent reports, the Urban Institute tried to quantify some of the potential impacts if the courts completely invalidate the law, as Trump and the coalition of Republican states are seeking.

It projected that the number of Americans without health insurance would rise from about 30 million under the ACA to about 50 million. That's an increase of almost two-thirds. Simultaneously, it projected that the amount of uncompensated care that hospitals and other providers must provide to patients without health insurance would soar by 82%, from about $60 billion under the ACA to over $110 billion without it.

Both of these dynamics would reverberate powerfully through the states that both sides see as the potential tipping points of the 2020 election. The Urban Institute forecast that the number of uninsured would more than double in Michigan and Pennsylvania and would rise by about two-thirds in Florida, two-fifths in North Carolina and Arizona, and a little over one-third in Wisconsin and Texas.

The amount of uncompensated care facing providers would roughly double in Pennsylvania and Michigan, rise by about 90% percent in Florida and increase by half or more in Texas, Arizona and North Carolina, the institute projected.

In a second study last month , the Urban Institute found that whites would account for nearly half of those who would lose coverage if the courts invalidate the ACA, while adults without four-year college degrees would compose more than four-fifths of the losers. Those numbers carry stark implications for the GOP at a time when it has grown increasingly dependent under Trump on the votes of whites without college degrees.

Issue of preexisting conditions affected 2018 election

It's not only coverage losses that might threaten key Trump constituencies if the ACA is repealed. The lawsuit would also invalidate the law's requirements that insurers provide coverage to patients with preexisting conditions at affordable costs. That poses the greatest risk to older and less affluent patients , including working-class whites, who tend to have the greatest health needs.

Political strategists in both parties agree that the promise to defend the ACA's guarantees for patients with preexisting conditions was critical to the Democrats' gains in the 2018 midterm elections. In exit polls, nearly three-fifths of voters said they believed Democrats would do a better job protecting patients with preexisting conditions -- and almost 90% of them supported Democratic candidates for the House.

The issue appeared especially important in propelling a modest but measurable Democratic recovery among working-class white voters, especially women. White women without college degrees have been a reliably Republican constituency in recent elections; their support in states such as Michigan, Wisconsin, Pennsylvania and Ohio was key to Trump's victory in 2016.

But in 2018, those women divided almost in half over whether Democrats or Republicans would do a better job protecting preexisting conditions, according to detailed exit poll results provided by Edison Research, which conducts the survey for a partnership of media organizations that includes CNN.

Fully 90% of the working-class white women who trusted Democrats more on that issue also voted Democratic for the House. And while working-class white men were less likely to trust Democrats to protect preexisting conditions (only about 40% did so), those who did broke overwhelmingly toward the party in the House elections. Democrats also enjoyed a commanding lead on the question among college-educated white men and women, as well as minority voters, Edison found.

Trump has shown that he believes it helps him with his base to be seen as fighting without reservation for conservative causes, whether he wins or not. In that sense, his camp believes he will benefit from challenging the ACA in court, even if the case fails, as most legal analysts still expect.

How the continuing Republican efforts to repeal the ACA affect 2020 will turn at least in part on what Democrats are proposing. The call by several leading Democratic contenders -- particularly Sens. Bernie Sanders, Elizabeth Warren and Kamala Harris -- to create a single-payer "Medicare for All" system that eliminates private insurance means that each of them, in effect, is also proposing to scrap the Affordable Care Act.

That prospect has opened perhaps the most important policy fissure in the Democratic Party. Liberal legislators and activists alike insist that a single-payer system is the sort of bold change that will inspire a massive turnout from young people and minorities who sat out the 2016 election.

But Democratic centrists worry that calling for replacing the ACA with an entirely new government-run system could alienate the same voters who have recoiled from the repeated GOP effort to repeal the law. That anxiety was compounded when the Democratic candidates in last month's debates all appeared to indicate that they would allow undocumented immigrants into the system, as Obama refused to do.

"Our message is if you work hard and have health insurance, not that we're going to improve it, we're going to take it away; but if you are undocumented we are going to give it to you," said former Chicago Mayor Rahm Emanuel, who led the drive for the ACA's passage as Obama's first chief of staff, in an interview.

Sanders, Warren and Harris are in the minority: Most of the Democratic field has rejected the idea of replacing the ACA with a single-payer system that eliminates private insurance. But those three are among the top four contenders in virtually all state and national polls. Among the top-tier candidates, former Vice President Joe Biden is the only one who has called for maintaining the ACA and rejecting single-payer.

Though he was somewhat muted in his opposition to the single-payer proposals during the recent Democratic debates, Biden denounced the idea more forcefully in his post-debate interview with CNN's Chris Cuomo. In excerpts airing Monday night, Biden sharply separated himself from the Democrats who would replace the ACA.

"Starting over would be, I think, a sin," Biden said.

At a moment when Trump has drawn a clear line by continuing his legal campaign against the law, "starting over" on the ACA would also represent a huge political gamble for Democrats.