If this seems to violate what you learned in “Schoolhouse Rock” about the roles of the three government branches, it should. Congress passed the Affordable Care Act in 2010, President Obama signed it into law, the Supreme Court upheld its constitutionality, and in the Trump administration Congress tried and failed to repeal it. How can Sessions effectively wipe the ACA off the books by telling his lawyers not to defend it?

As Sessions acknowledged, the Justice Department has a “long-standing tradition of defending the constitutionality of duly enacted statutes if reasonable arguments can be made in their defense.” This is best understood as a norm rather than a legal or constitutional duty. Norms are customs or practices that serve the ends of constitutional governance, with their force coming from long-standing adherence by both sides of the political spectrum. Although it may be in the short-term interest of the side holding power at any moment to violate the norm, each side knows that the other will one day take power again, and that a world governed by the norm is better than a world without it.

Unfortunately, the Obama administration departed from that norm in 2012 by abandoning the Defense of Marriage Act in the Supreme Court — winning praise, ironically, from many of the same people who are condemning Sessions’s decision. The Trump administration’s similar action probably renders the norm defunct. Sessions called the Texas litigation against the ACA the “rare case” in which the norm should not be followed. But now that both the Obama and the Trump administrations have turned their backs on the practice, refusal to defend will almost certainly become the new norm. Both sides will regret this.

The refusal of government lawyers to defend statutes upsets the balance among the executive, the legislative and the judiciary branches. It enables the executive branch effectively to repeal a law without congressional vote and to be the final decision-maker in constitutional litigation. It portends what James Madison in Federalist No. 48 called the “tyrannical concentration of all the powers of government in the same hands.” As the Supreme Court opinion noted when the Obama administration refused to defend the Defense of Marriage Act:

“When Congress has passed a statute and a President has signed it, it poses grave challenges to the separation of powers for the Executive at a particular moment to be able to nullify Congress’s enactment solely on its own initiative and without any determination from the Court.” (United States v. Windsor (2013)).

In some cases, there will be other parties in the litigation with standing to defend the statute, but this is purely a matter of happenstance. In the California same-sex marriage case a few years ago, there was no such party. In the Texas ACA litigation, 16 states and the District of Columbia have been permitted to intervene in support of the statute, but whether they have constitutional standing is highly questionable. In many cases, no one other than the government has standing to defend, and the statute will be invalidated without a final judicial decision on the merits. In effect, the executive branch will be the final word — and there is nothing Congress or the Supreme Court can do about it.

The established norm of defending the constitutionality of congressional acts arose from the fact that the Justice Department is a law office with a single client: the people of the United States. When the people’s representatives in Washington have enacted a law, they have a legitimate interest in seeing the law carried out unless and until it is repealed. If the law has a reasonable basis in the Constitution, the attorney general, as lawyer for the people, has an ethical obligation to defend it in court, just as any lawyer has the obligation to zealously defend his client’s interests even if his own theories of the law might differ. It should not matter that the president himself has concluded the statute is unconstitutional. President Trump is not Sessions’s client. He is Sessions’s supervisor — which means that he has the ultimate constitutional obligation to ensure that Sessions is faithfully executing his responsibilities. In this case, that means defending a statute even if both Trump and Sessions think it should be held unconstitutional.

Michael W. McConnell is the Richard and Frances Mallery professor of law and director of the Constitutional Law Center at Stanford Law School and a senior fellow at the Hoover Institution. He formerly served as a judge on the U.S. Court of Appeals for the 10th Circuit.