Last week, the Justice Department notified a federal appeals court that it now agrees with a district judge’s December ruling that because the Affordable Care Act’s (ACA) individual mandate is unconstitutional, the entire act is void.

It has been widely reported that the Justice Department’s new view is “a significant shift” from its earlier argument—“that the penalty for not buying insurance was legally distinct from other provisions of the law, which could still stand.”

For decades, the Justice Department has held the view that it has a duty to defend an act of Congress when reasonable arguments can be made in support of the law. With limited exceptions, it is an abandonment of norms to disregard this obligation. If the executive branch were, for example, to refuse to defend an environmental law, it could effectively undermine Congress’ ability to enact laws in the first place. The decision to not defend an act of Congress is almost always a mistake—no matter the president.