The Justice Department said last night that the Affordable Care Act's individual mandate has become unconstitutional. Even more importantly, it says the law's most popular provision — guaranteed coverage for people with pre-existing conditions — also has to go.

Threat level: The legal challenge at issue here may be somewhat of a long shot on the merits. But if it does ultimately succeed, it could toss out the hardest-fought and most significant parts of the ACA.

Why it matters, norms edition: When people challenge a federal law in court, DOJ defends that federal law, almost every time.

No, this is not the first time in history DOJ has done something like this. The Obama administration's Justice Department quit defending the federal ban on same-sex marriage. But a thing does not need to be literally unprecedented to be a big deal, and this is a big deal.

"The Department of Justice has a duty to defend federal laws when reasonable arguments can be made in defense of the law. I find it impossible to believe that the many talented lawyers at the Department could not come up with any arguments to defend the ACA’s insurance market reforms," said Donald Verrilli, who defended the ACA before the Supreme Court as Obama's solicitor general. "This is a sad moment."

Why it matters, health policy edition: We're used to having complex arguments about how this complex law would function without the individual mandate.

But if insurers can simply deny sick people coverage again, then it's not a matter of how the health of the risk pool affects premiums. They can just be excluded from the market altogether — rolling back the ACA's most popular and foundational promise.

What's next: To be clear, DOJ has not won this argument. It has only made the argument. But its position carries weight in the courts, and this case will work its way up through conservative courts.

It's being heard by a conservative judge, Reed O'Connor. His ruling will inevitably be appealed to the 5th Circuit Court of Appeals — a conservative court. From there it would go to the Supreme Court.

And there, it'd once again come down to two Republican-appointed justices: John Roberts and, if he's still on the court, Anthony Kennedy.

The politics: Democrats are already arguing that this plays right into their hands in the midterms. There's a logic to that — "Trump is trying to take away coverage for pre-existing conditions" is a pretty obvious hit.

"Health care is the leading issue for voters this election year, and Republican Senate candidates will have to defend this decision along with every other plank of their deeply unpopular agenda," Senate Democrats' campaign committee said last night.

But even Democrats' best-case midterm scenarios have very little chance of affecting the outcome of this case. It's already in motion, and winning the House, or even the Senate, won't force the Trump administration to change this decision.

Refresher: How this all works:

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.

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.

That's the argument DOJ signed onto last night.

Key difference: The states say the whole law must fall along with the mandate. DOJ says it's just two provisions that have to go — requiring insurers to cover sick people, and preventing them from charging individuals a higher premium because of their pre-existing conditions.

The other side: A group of Democratic attorneys general has been granted permission to defend the ACA in this case, so it will have an ally in court — just not the federal government.