Justice Samuel Alito, a former U.S. attorney, said his experience was that many magistrates were "unreceptive to receiving warrant applications in the middle of the night" making it impractical in many jurisdictions to secure a timely order.

Steven Shapiro, the legal director of the American Civil Liberties Union who represented McNeely, said there was "no doubt" that using a needle triggers a warrant requirement.

Neither states nor criminal suspects in general should expect a clear ruling in their favor. It is more likely the court will issue a nuanced opinion.

A victory allowing automatic blood draws would not stop other states from maintaining strict standards for their use. Indeed, the simple threat of a needle may be incentive enough for suspected drunk drivers to agree to less invasive breath tests.

Both sides concede that dynamic already exists in many cases.

But if McNeely prevails, police may be required to adopt more extensive guidelines on testing policies, narrowing the emergency exceptions to the warrant requirement.

The practical effect may be to force police to streamline and speed up the warrant process, so that drunk drivers are tested before it is too late to preserve the evidence.

The case is Missouri v. McNeely (11-1425). The ruling is due by the spring.