Most, if not all, states have adopted “implied consent” laws where drivers may be tested if police have probable cause to suspect they have been driving while intoxicated. Drivers may withdraw consent and refuse to take a test, subject to penalties. In Birchfield v. North Dakota (2016) the Supreme Court held that generally police must obtain a warrant to require a blood test (versus a breath test) where officers have probable cause.
But what if a driver is unconscious and unable to withdraw consent to a blood test (and unable to take a breath test)? Wisconsin and 28 other states allow warrantless blood draws of unconscious drivers where police have probable cause to suspect drunk driving.
The question the Supreme Court will decide in Mitchell v. Wisconsin is whether a statute authorizing a blood draw from an unconscious motorist provides an exception to the Fourth Amendment warrant requirement.