Attorney General Jeff Sessions says the Trump administration will not defend the constitutionality of parts of the Affordable Care Act. (Jabin Botsford/The Washington Post)

The Trump administration’s decision not to defend parts of the Affordable Care Act against a constitutional challenge from Republican-led states is only the most dramatic of a number of shifts the Department of Justice has taken to distance itself from the previous administration.

The administration has changed the department’s position on voting rights cases, switched sides in a suit challenging Texas’s redistricting plans, revoked past support for organized labor, rescinded protection for transgender students and split with the Equal Employment Opportunity Commission on workplace protection for gays. Some of the issues are under consideration this term at the Supreme Court.

“How many times this term already have you flipped positions from prior administrations?” Justice Sonia Sotomayor asked President Trump’s Solicitor General Noel J. Francisco during oral arguments in February in a case on union fees for public employees.

But the decision not to defend part of the ACA is likely to be the most controversial. It is rare, but not unprecedented, that an administration says that a law passed by Congress is unconstitutional. Attorney General Jeff Sessions said the decision was taken only after consultation with Trump.

It is even rarer when the law in question was passed only last year and supported by President Trump.

The filing comes in a case brought by Texas and 19 other states and filed in federal court in Fort Worth. It challenges the constitutionality of the ACA because of changes made in the tax-cut bill passed by the Republican-led Congress and signed by Trump at the end of 2017.

The DOJ brief filed in the case brought immediate protests from those who were instrumental in the legal fight that led the Supreme Court to uphold President Barack Obama’s signature domestic achievement in 2012.

“The Department of Justice has a duty to defend federal laws when reasonable arguments can be made in defense of the law,” said Obama’s former solicitor general, Donald B. Verrilli, who argued the ACA case at the high court. “I find it impossible to believe that the many talented lawyers at the department could not come up with any arguments to defend the ACA’s insurance market reforms, which have made such a difference to millions of Americans.”

In fact, three career DOJ lawyers who had been working on the case withdrew from it just before the department’s filing Thursday night.

Others were quick to remember the last time the Justice Department made such a dramatic move — during the Obama administration.

The department said it would not defend the Defense of Marriage Act (DOMA), passed in 1996, which said that the federal government recognized marriage only as between one man and one woman, and that states did not have to recognize same-sex marriages conducted elsewhere.

The Supreme Court, on a 5 to 4 vote in 2013, agreed that the law was unconstitutional.

“You can argue that what Obama did was right and what Trump is doing is wrong, but Obama doing it made it possible for Trump,” said William Baude, a University of Chicago law professor.

Even as Sessions declared in a letter to Congress that this was “a rare case where the proper course is to forgo defense,” his office released a list of instances going back to 1979 when the department declined to defend an initiative’s constitutionality.

Adam J. White, executive director of the C. Boyden Gray Center for the Study of the Administrative State at George Mason University’s Antonin Scalia Law School, said each case has its own unique facts.

While cautioning that he was not judging the merits of the Justice Department’s position, he said it was “a little bit rich” for liberals to criticize Sessions’s decision to say he would not defend the constitutionality of parts of the law.

Besides DOMA, the Obama administration chose not to enforce some immigration laws, White said, and even part of the ACA “when it was inconvenient.”

Others have been outspoken in criticizing the legal reasoning in the Justice Department’s brief, and said the withdrawal of the three career attorneys can only be seen as a protest of the filing. A Justice Department official said only: “The department decided to sub in a new team for a new legal position.”

The states mounting the legal challenge argue that tax law cut the penalty for not having insurance to zero, thus rendering the “individual mandate” toothless. Because the requirement as of Jan. 1, 2019 won’t raise money, that undoes its role as a “tax,” the states claim. And because a scant majority of the Supreme Court had said it was Congress’s taxing powers that saved the ACA from being unconstitutional, that protection was no longer there. The entire law must fall, they say.

The Justice Department did not go that far. But it did say that because the individual mandate is unconstitutional, so is another, more popular part of the law: protections for people with preexisting conditions. The brief says that part of the law cannot be severed from the mandate.

The question for a court in such instances is whether Congress wanted one part of the law to continue even if another part is struck down. Proponents of the law contend that is exactly what Congress was saying in 2017, when it undermined the individual mandate, but left the rest of the law in place.

Liberal law professors Ian Samuel and Leah Litman wrote on the legal blog Take Care that the court challenge is an invitation to the courts to do what Republicans in Congress were unable to accomplish.

“We all know that the leadership of the Republican Party would be happy to see the Affordable Care Act repealed; we also all know they couldn’t actually get the votes together to do it,” the two write, adding that the message to the court is, “Here’s where you come in.”

Nicholas Bagley, a University of Michigan law professor and ACA supporter, made a similar point about severability.

“We don’t have to speculate what Congress would’ve done if it had a choice between invalidating the ACA’s insurance reforms or just invalidating the mandate,” he wrote in a blog post. “Congress made that choice. For a court to now reject it in the service of an absurd argument about severability would be the rankest kind of judicial activism.”

For Verrilli, at least, that is what makes the current decision not to defend part of the law different from the Obama administration’s decision not to defend DOMA, passed years before.

“We know for certain that Congress wanted the ban on discriminating against people with preexisting conditions to survive even without the insurance mandate” because that is what Congress enacted, he said. To argue otherwise, he said, “flatly contradicts the statute that Congress enacted and the president signed into law.”