LAWMAKERS GRILLED Attorney General William P. Barr this week on the Russia investigation, the report of special counsel Robert S. Mueller III and the possibility that President Trump obstructed justice. But for all the attention concentrated on those issues, the most concerning thing Mr. Barr said had nothing to do with Russia, but his defense of the Justice Department’s indefensible move to strike down Obamacare.

Mr. Barr entered office promising to respect Justice Department norms. But he apparently has few qualms about eroding an essential one: that government lawyers defend all duly passed laws in court as long as there is a reasonable argument in their favor. This standard ensures that laws passed by Congress and signed by the president are capably defended from legal challenges regardless of who sits in the White House. The alternative is chaos: Laws the president likes get defended rigorously, while statutes the White House disfavors are attacked by the executive branch.

In the case of Obamacare, Mr. Barr’s Justice Department joined challengers to the health-care law in insisting that the whole program must go. This shifts the department’s earlier position that judges should strike only a portion of the law and uphold the rest. The legal reasoning behind the call is embarrassingly thin, considering that Congress’s intent, on which the case turns, is clear — and that even if one took the challengers’ underlying claims seriously, it would still be illogical to quash the whole law.

A coalition of states is defending Obamacare, so the Justice Department’s hostile position in the suit might not matter much. But the department’s take might still matter to judges considering how to rule. Moreover, imagine a case in which there are fewer willing defenders of the law, particularly with the resources to mount a strong defense. These are among the reasons the Justice Department should defend all laws except in extremely rare circumstances — such as the Obama administration’s decision not to defend the bigoted Defense of Marriage Act.

Mr. Barr this week sought to apply a lower standard to the Justice Department. He said the attorney general should give the president his best legal advice. But if the president decides that he prefers a different position, “the attorney general litigating on behalf of the United States should take that position if it’s reasonable and a defensible legal position, even if it’s not the position that the attorney general would take if the attorney general was a judge.” So, instead of the Justice Department defending laws unless there is no plausible legal argument in their favor, Mr. Barr said the standard is that the president should decide what to do unless there is no plausible legal argument in his favor.

It should be noted that even under this standard, the Justice Department’s stand on Obamacare still fails. There simply is no reasonable argument for the department’s position in the case. Yet even if its legal merits were greater, the department’s stance would still be a mistake.

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