By Jonathan Stempel

WASHINGTON, Jan 9 (Reuters) - Missouri urged the U.S. Supreme Court on Wednesday to let police take involuntary blood samples from suspected drunken drivers without a warrant, something that Chief Justice John Roberts said evoked a "pretty scary image" of government power.

The case was one of two involving the privacy rights of drivers argued before the nation's highest court on Wednesday. A South Carolina case addressed whether lawyers can gather personal information on drivers from state databases to help find possible plaintiffs for lawsuits.

While the oral argument in the Missouri case did not produce a clear consensus among the nine justices, their decision could shape motorist privacy rights against unreasonable searches and seizures under the Fourth Amendment to the U.S. Constitution.

"It's a pretty scary image of a person being restrained ... and a representative of the state approaching him with a needle," Roberts said.

The court regularly lets the government obtain evidence in criminal cases without warrants. But Missouri's case against defendant Tyler McNeely is notable because the evidence against him was inside his body - specifically, his level of intoxication.

"How can it be reasonable to forgo the Fourth Amendment in a procedure as intrusive as a needle going into someone's body?" Justice Sonia Sotomayor asked John Koester, a lawyer representing Missouri.

McNeely's blood test was administered after he had performed poorly in sobriety tests following a traffic stop and refused to submit to a breath test.

A Missouri highway patrolman then took McNeely to a hospital. There, some 25 minutes after the stop, and while handcuffed and restrained, McNeely had his blood-alcohol content measured at 0.154 percent, nearly twice the state's legal limit.

The Missouri Supreme Court suppressed the blood sample, finding no "special facts" or "exigent circumstances" to justify it without a warrant.

But the state said blood tests are an effective way to obtain evidence of drunken driving. It said this evidence could vanish in the time it took to get a warrant because alcohol quickly dissipates in the bloodstream.

"Evidence is being lost at a significant rate for every minute that passes," Koester told the justices.

STICKING A NEEDLE IN THE ARM

But the justices noted that sticking a needle into a suspected drunken driver's arm is not the same as taking a urine sample, giving him a breathalyzer test, or ordering him to walk in a straight line, touch his nose, and stand on one foot. All can yield evidence of drunkenness, but vary in intrusiveness.

"What's reasonable for sticking a needle in your arm is not the same as what's reasonable for asking you to blow up a balloon" in a breath test, Justice Antonin Scalia said.

McNeely's lawyer Steven Shapiro noted that about half of all U.S. states require warrants for blood samples, and Justice Anthony Kennedy suggested that their apparent success in thwarting drunken driving might be a factor in the case.

According to federal data, 1.22 million people were arrested in the United States in 2011 for driving under the influence, and one person dies every 51 minutes nationwide as a result of drunken driving crashes.

Koester said that if the court authorized warrantless blood tests, it would encourage more suspected drunken drivers to accept the less intrusive breathalyzer tests.

Scalia told Nicole Saharsky, a U.S. Justice Department lawyer supporting Missouri's view, that there might be no need to go so far.

"I don't know why it isn't adequate to produce that result simply to put the drunk driver in a paddy wagon and on the way to the hospital say, 'You know, we're going to be in the hospital in 20 minutes; we're applying for a warrant; when we get there, we're going to stick a needle in your arm, unless, of course, you agree to take the breathalyzer test,'" he said.

Saharsky responded that such suspects would likely "take their chances" and hope that a delay in testing would allow alcohol in their bodies to dissipate over time.

Justice Elena Kagan wasn't so sure. "Maybe they're drunk," she said, prompting courtroom laughter.

The second case concerned the application of the Driver's Privacy Protection Act, a federal law that in general blocks state motor vehicle bureaus from disclosing information about drivers, including names and contact information.

But exemptions include a "litigation exception" allowing disclosures to help a lawyer with an "investigation in anticipation of litigation."

Wednesday's case was brought by three drivers against lawyers who they said had obtained personal information from the South Carolina Department of Motor Vehicles without consent, in a bid to find plaintiffs for litigation against car dealerships.

A federal appeals court allowed these disclosures. Joseph Guerra, who represents the drivers, told the Supreme Court this ran afoul of Congress' desire to prevent "undue public/private access to DMV (department of motor vehicle) information."

Paul Clement, who argued on behalf of the defendant lawyers, said the federal law was not an "all-purpose privacy statute," and that its broad language covered his clients' activity.

The Supreme Court is expected to decide both cases by the end of June.

The cases are Missouri v. McNeely, U.S. Supreme Court, No. 11-1425; and Maracich et al v. Spears et al, U.S. Supreme Court, No. 12-25.

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