The Trump administration’s decision not to support the Affordable Care Act in a federal appeals court at yet another moment in the law’s extended existential crisis has refocused attention on the Justice Department’s “duty to defend” acts of Congress.

The phrase “duty to defend” has the gravitas of America’s founding era. But legal scholars Neal Devins and Saikrishna Prakash trace its first use by the Justice Department to a lengthy memo written in 1980 by then-Attorney General Benjamin Civiletti, responding to unfriendly questions from Sen. Strom Thurmond (R-S.C.) “It has no constitutional basis,” Devins said in an interview.

Since then, the “duty to defend” has taken on a sacred status among career Justice Department lawyers and generally within the legal profession, as demonstrated by some of the reactions to Trump’s decision. “It’s a shocking dereliction of the Justice Department’s duty,” University of Michigan law professor Nicholas Bagley wrote in the New York Times; it “crosses a new legal Rubicon,” said Josh Blackman, of South Texas College of Law Houston, writing in The Washington Post.

But how sacrosanct is the duty to defend in practice? That’s another question, one that was confronted in 2011, by President Barack Obama and his attorney general, Eric H. Holder Jr. A law called the Defense of Marriage Act, or DOMA, which defined marriage for federal purposes as between a man and a woman, faced a serious constitutional challenge in court. Neither approved of the 1996 law, and it was politically disadvantageous to be seen as defending it while the 2012 election approached.

After considerable debate within the department — and after reportedly overruling career officials — Obama and Holder made the call. Holder notified Congress that he and the president now believed the law unconstitutional and would not defend it in court.

Democrats, including then-House Minority Leader Nancy Pelosi (D-Calif.), praised the move.

But it did not sit well with Republicans then in Congress, including Rep. Mick Mulvaney of South Carolina. Mulvaney, along with Reps. Mike Pence (R-Ind.) and Mike Pompeo (R-Kan.) joined other conservatives in signing a letter of protest to Obama.

“As President, you and the Department of Justice (DOJ) have a duty and responsibility to defend the laws of the United States,” they wrote. “While it may be inevitable that you, or any other administration, may disagree with a bill signed into law by previous Presidents, that does not negate your responsibility to uphold those laws.”

Mulvaney also co-sponsored a “sense of Congress” resolution condemning the administration for “discontinuing to defend the Defense of Marriage Act.”

But it didn’t take long to find out how sacrosanct the move was for Mulvaney.

Fast-forward eight years. Now President Trump and Mulvaney, as his chief of staff, were faced with the question of whether to defend from a court challenge a law they loathed, Obama’s Affordable Care Act. Trump had promised to get rid of Obamacare and with the 2020 election approaching, Congress wasn’t about to repeal it, at least not in its entirety.

In a lawsuit initiated by Texas Attorney General Ken Paxton (R), an ardent Obamacare opponent, U.S. District Judge Reed O’Connor ruled in December that because the Republican-controlled Congress in 2017 eliminated the tax penalty that went with the Obamacare mandate — the constitutional taxing power linchpin that prompted the U.S. Supreme Court to uphold the law — at least part of the law was now unconstitutional.

In some cases, judges will let the rest of a law stand when part of it falls, “severing” the valid from the invalid. But O’Connor declined to do so, ruling that the entire edifice had to go.

Like Obama, according to reports in The Washington Post and the New York Times, Mulvaney and Trump heard arguments from the Justice Department, which reportedly raised the duty to defend. And like Obama, they dismissed them and decided not to defend the law on the grounds that the ACA was unconstitutional, notifying the U.S. Court of Appeals for the 5th Circuit, where the challenge is being considered, on Monday. (The ACA will be defended by, among others, a California-led coalition of states, as well as the District of Columbia.)

According to a 2012 Columbia Law Review article by Devins, a William and Mary law professor, and Prakash, of the University of Virginia Law School, the Department of Justice declined to defend roughly 75 acts of Congress between 1976 and 2011. Few of these decisions generated much publicity or political controversy. Many of the undefended laws, they write, were essentially dead letters, already clearly at odds with Supreme Court precedent with little or no chance of the court changing its mind.

They describe nine of the cases as “high-profile refusals to defend.”

In 1990, for example, the George H.W. Bush administration asked the Supreme Court in Metro Broadcasting, Inc. v. FCC to strike down congressionally mandated minority preferences for broadcast licenses. The court upheld the preferences nevertheless.

In 1994, President Clinton’s solicitor general, Drew Days, refused to defend the conviction under a federal child pornography law of a Pennsylvania man. After an outcry from conservatives and a nonbinding resolution from the Senate critical of Days, Clinton and then attorney general Janet Reno reversed course. Ultimately, the Supreme Court declined to review the case. The U.S. Court of Appeals for the 3rd Circuit reaffirmed the man’s conviction in United States v. Knox.

The title of Devins’s and Prakash’s law review article —“The Indefensible Duty to Defend” — hints at where they stand.

Devins said he sees the concept as “self-serving” institutionally for the Justice Department, a way to exercise power over litigation.

“Legal policy changes” as administrations change, Devins said. In most cases, an administration “may say it’s the rule of law that’s compelling them” not to defend a statute, he said, but in fact “it’s usually a change of policy. The government should be more honest and shouldn’t pretend that it’s based on an inability to advance a legitimate legal argument.”

Others, like Bagley and Blackman, say the duty to defend acts of Congress is crucial to the Justice Department’s credibility in the courts.

Some evidence of this has come from the highest court itself.

The Supreme Court validated the Obama-Holder position in DOMA in United States v. Windsor, declaring unconstitutional the law’s prohibition on federal recognition of legally married same-sex couples.

But even as it did, Justice Anthony M. Kennedy, writing for the majority on June 26, 2013, chastised the Obama administration, describing its decision not to defend DOMA as “unusual” because it was based not on a court ruling but on “the Executive’s own conclusion.”

Justice Antonin Scalia, on the other side in the 5-to-4 ruling, agreed. It’s one thing for the executive branch to refuse to defend a law — such as the legislative veto struck down by the court in 1983 — because it usurps presidential power. But in a case like DOMA, which does not involve separation of powers, “There is no justification for the Justice Department’s abandoning the law,” Scalia wrote.

The duty to defend is a norm of governance, not a law, and when it comes to norms, few are enduringly sacred. Every president who departs from tradition gives license to his successors to do the same, until the norm is no longer the norm.