The Supreme Court has handed down an important Fourth Amendment case, Birchfield v. North Dakota, on the government’s power to compel people arrested for drunken driving to take breath tests and blood tests.

The upshot: The government can require a person arrested for drunken driving to take a warrantless breath test, but ordinarily the government cannot require a person to submit to a blood draw without first obtaining a warrant. The court reaches this result under the “search incident to arrest” exception. The distinction between blood tests and breath tests is justified by the different invasiveness of the two kinds of tests that makes warrantless breath tests reasonable but warrantless blood tests ordinarily unreasonable.

Justice Samuel Alito wrote the majority opinion, joined by Chief Justice John Roberts and Justices Anthony Kennedy, Stephen Breyer and Elena Kagan. Justices Sonia Sotomayor and Ruth Bader Ginsburg would have held that both kinds of tests ordinarily require a warrant; Justice Clarence Thomas would have held that neither test requires a warrant.

I’m at a meeting all day and won’t have time to blog extensively on Birchfield until later. Based on a quick read, however, I think it’s a sound opinion. This was a really tricky Fourth Amendment case. It was like a criminal procedure exam in which the problem raised a bunch of different doctrines without obvious ways to work through the thicket of them. I think the majority did a pretty good job dealing with a very hard problem.

More later, when I have had a chance to read the decision with more care and to reflect on it at more length.