PRESIDENT TRUMP humiliated the Justice Department this week, not with something he tweeted but in a move that could be far more consequential. According to Politico, the White House ordered the department to change its position on a major anti-Obamacare lawsuit — from weird to truly wacky — despite opposition from Attorney General William P. Barr. Indeed, Mr. Barr should be embarrassed: The legal reasoning at work in the decision is poor, and the erosion of Justice Department norms is worrying.

The lawsuit, the latest in a string of desperate and misguided legal challenges to Obamacare, contends that the Republicans’ 2017 tax-cut bill undercut the health-care law so fundamentally that the law can no longer stand. The 2017 bill dropped to zero the penalty for defying the individual health-insurance mandate, which requires Americans to maintain health-care coverage. Because the health-insurance mandate no longer operates like a tax — the basis on which the Supreme Court upheld it — it is no longer legal, goes the argument. And because the mandate was an important piece of Obamacare’s design, Congress would not have wanted the law to function without it, and the whole law must go. Previously, the Trump administration argued that only some of the law would have to be quashed. Now, the Justice Department says the whole thing must disappear.

This is bonkers. Obamacare is a sprawling law that includes big changes in Medicaid, Medicare, hospital regulation and public-health policy that have no relationship with the individual mandate. The loss of the mandate poses no threat to the viability of Obamacare’s expanded Medicaid program, on which 17 million Americans rely. It has no bearing on whether restaurants should have to post calorie counts on their menus or whether payments to Medicare providers should be cut. Mandate or no, these reforms could persist.

Congress imposed the mandate to help the highly regulated individual insurance markets it sought to create for people who were not on Medicaid or employer plans. Insurance companies could not turn away or upcharge people with preexisting conditions, but in return they got a wave of new customers compelled to buy coverage. One can debate how important the mandate is for the proper functioning of the law. But Congress in 2017 neutered the mandate and left the rest of the law in place; the Justice Department cannot argue seriously that Congress did not intend for the rest of the law to stand without the mandate if that is precisely what Congress chose to do.

The legal reasoning behind the Justice Department’s new position is so flawed, it is hard to imagine any appeals court agreeing. Yet even if this Obamacare challenge gets the rude dismissal it deserves, the Trump administration has set an awful precedent, siding with anti-Obamacare cranks and refusing to defend a law of the United States, based on nothing but partisan disagreement with the policy. The Justice Department is supposed to defend the law unless there is no reasonable argument in its favor. In this case, the department has refused to defend the law when there is no reasonable argument against it.

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