The Justice Department last week stopped short of fully agreeing with that rationale, saying that some of the law’s provisions, like the Medicaid expansion that brought coverage to nearly 12 million low-income Americans, could stand without the penalty, but that the court should strike down key consumer protections at the law’s core.

Such protections prohibit insurers from denying coverage to people with pre-existing medical conditions or from charging such people more for coverage. The rules, which protect some 52 million Americans, according to a Kaiser Family Foundation analysis, are far and away the law’s most popular features; a 2016 poll by the same organization found that 75 percent of Democrats and 63 percent of Republicans support them. But, in a letter to the House speaker, Paul Ryan, Attorney General Jeff Sessions said his department could think of no rational argument with which to defend them.

They’re not thinking hard enough. Mr. Sessions would do well to read the legal filings of 16 Democratic state attorneys general who have stepped in to do his job for him. Led by Xavier Becerra of California, they make a persuasive case, echoing sentiments expressed by legal observers on both sides, that the suit is absurd.

Mr. Sessions’s supporters have compared his stance to the Obama administration’s refusal to defend the Defense of Marriage Act in 2011. But the two aren’t equivalent. Challenges to the marriage law raised fundamental questions about who we are as a people, and what we want the Constitution to stand for. The Justice Department made the case, later accepted by the Supreme Court, that American society had evolved to the point where denying federally protected marriage rights to same-sex couples was discriminatory and therefore unconstitutional.

The Affordable Care Act, by contrast, has withstood numerous legal challenges in the eight years since it became law, including two at the Supreme Court. The current lawsuit against it hinges not on big questions of equality and justice, but on a technicality.