The Justice Department will not defend the Affordable Care Act in court, and says it believes the law's individual mandate — the provision the Supreme Court upheld in 2012 — has become unconstitutional.

Why it matters: The Justice Department almost always defends federal laws when they're challenged in court. Its departure from that norm in this case is a major development — career DOJ lawyers removed themselves from the case as the department announced this shift in its position.

The details: The ACA's individual mandate requires most people to buy insurance or pay a tax penalty. The Supreme Court upheld that in 2012 as a valid use of Congress' taxing power.

When Congress claimed it repealed the individual mandate last year, what it actually did was drop the tax penalty to $0.

So the coverage requirement itself is still technically on the books. And a group of Republican attorneys general, representing states led by Texas, say it's now unconstitutional — because the specific penalty the Supreme Court upheld is no longer in effect.

The Justice Department agreed with that position in a brief filed Thursday night.

DOJ said the courts should strike down the coverage requirement, as well as the provision of the law that forces insurance companies to cover people with pre-existing conditions.

Between the lines: For the Justice Department to stop defending a federal law is not unprecedented — the Obama administration did it with the Defense of Marriage Act. But it is exceptionally rare.

Yes, but: A group of Democratic attorneys general has been granted permission to defend the ACA in this case, so someone will be in its corner.

What to watch: The argument against it is by no means a slam dunk. For starters, critics — now including the Justice Department — will have to prove that people are still being injured by the remaining shell of the individual mandate, even without a penalty for non-compliance.

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This story has been updated to note that the Obama administration stopped defending the Defense of Marriage Act.