WASHINGTON – The Supreme Court justices sounded wary Wednesday of giving the police a free hand to forcibly take blood from motorists suspected of drunken driving.

“It’s a pretty scary image of somebody restrained, and a representative of the state approaching them with a needle,” said Chief Justice John G. Roberts Jr.

While the police stop swerving drivers at all hours of the day and night, rarely are motorists required to undergo a blood test. Typically, an officer tells a driver who appears to be drunk to get out of the vehicle, walk a straight line and recite the alphabet. If he fails those tests, the motorist is asked to blow air into a device that measures alcohol in the blood.

A blood test is a fall-back measure, an option for when a driver refuses to take a breath test. But the law in half the states and in many court jurisdictions says an officer must obtain a search warrant before he can force a motorist to have a needle stuck in his arm.

Seeking a change that could have a national impact, Missouri prosecutors and a Justice Department lawyer urged the high court to set aside the requirement for a search warrant. They argued that because alcohol dissipates in the blood, a police officer need not take the time to check with a magistrate.

“Quickly securing blood alcohol evidence with as little delay as possible is incredibly important,” John Koester Jr., a Missouri state attorney told the justices. It takes 30 minutes to two hours to obtain a search warrant, he said.

But in their questions and comments, most of the justices said they were reluctant to set aside the warrant requirement in routine drunken driving cases.

As Justice Sonia Sotomayor said, the Fourth Amendment’s ban on “unreasonable searches” has limited the power of officials to order highly personal probes. “So how can it be reasonable to forego the Fourth Amendment in a procedure as intrusive as a needle going into the body?” she asked.

Sotomayor, who regularly injects herself with insulin to treat her diabetes, worried too about the impact of a court decision in favor of blood tests. Most jurisdictions rely on breath tests. A ruling “by us in your favor is going to change that” and encourage routine use of blood tests, she told the Missouri prosecutor.

Koester disagreed. “I think the end result will be more people will agree to take a Breathalyzer test,” he replied.

“Why don’t you force him take the Breathalyzer test, instead of forcing him to have a needle shoved in his arm?” asked Justice Antonin Scalia.

Because the suspect must be willing to blow his breath into the device, Koester replied. “It’s sort of like you can put a balloon in front of somebody’s mouth, but you can’t make him blow it up,” he said.

The case before the court, Missouri v. McNeely, started when a state patrolman stopped Tyler McNeely for speeding about 2 a.m. on October 3, 2010. The motorist failed the roadside sobriety tests and refused to take a breath test.

The officer took him to a hospital, and without stopping to request a search warrant, he told an orderly to take a blood sample from McNeely. A lab reported his blood alcohol level was .154 percent, nearly double the legal limit of .08 percent. The Missouri Supreme Court threw out the blood test evidence, ruling the officer should have obtained a search warrant.