WASHINGTON -- A national drive to halt the death penalty met defeat at the Supreme Court on Wednesday when the justices ruled that lethal injections, if properly administered, were a humane means of executing a condemned prisoner.

By a surprisingly large 7-2 margin, the court rejected a constitutional attack on the main method of carrying out the death penalty across America. Its ruling cleared the way for executions to resume in several states after a seven-month delay.

Since October, officials and judges in those states -- including California -- have put executions on hold while awaiting the outcome of the Kentucky case decided Wednesday. Gov. Arnold Schwarzenegger said the ruling "supports California's lethal-injection protocol" and should allow executions in the state to resume.

The court's opinion, by Chief Justice John G. Roberts Jr., confirmed that there was strong support for the death penalty among the justices, and an unwillingness to tolerate endless delays.

"We begin with the principle . . . that capital punishment is constitutional. It necessarily follows that there must be a means of carrying it out," Roberts wrote.

"Some risk of pain is inherent in any method of execution -- no matter how humane -- if only from the prospect of error in following the required procedure."

Roberts said the court would not allow a theoretical risk that a future execution could be botched to stand in the way of carrying out the death penalty.

He also set a high bar for future challenges to carrying out the death penalty. To halt an execution, defense lawyers must show that there is a "substantial risk" that the condemned prisoner will suffer "severe pain," the chief justice said. And they have yet to provide such evidence, he added.

"A state with a lethal-injection protocol substantially similar to the protocol we upheld today would not create a risk that meets this standard," he said.

Agreeing with Roberts, Justice Samuel A. Alito Jr. added a note to say that the court should not allow "litigation gridlock" to "produce a de facto ban on capital punishment." Justice Anthony M. Kennedy also agreed with Roberts.

Justices Clarence Thomas and Antonin Scalia said they would go further and reject all challenges to an execution method unless it was "deliberately designed to inflict pain."

Despite the lopsided outcome of this case, the Supreme Court remains deeply split on capital punishment. Death penalty cases that come before the high court often are decided by a 5-4 vote.

Justice John Paul Stevens, who will be 88 on Sunday, said his three decades on the court had convinced him that the death penalty should be ended.

He said he agreed with the late Justice Byron White, who once described capital punishment as "the pointless and needless extinction of life with only marginal contributions" to society.

Nonetheless, Stevens voted with Roberts to reject the challenge to lethal injections, since there was no evidence that Kentucky's approach was badly flawed. Justice Stephen G. Breyer agreed for much the same reason.

Justices Ruth Bader Ginsburg and David H. Souter stood alone in dissent. They said they would maintain the hold on executions because Kentucky "lacks the basic safeguards" to ensure that an inmate dies a painless death.

"I would not dispose of this case so swiftly, given the character of the risk," Ginsburg said.

Since the 1970s, most of the 36 states that carry out the death penalty have abandoned electrocutions or the gas chamber and switched to lethal chemicals. Most rely on a three-chemical mix that includes an anesthetic, a paralyzing drug and a heart-stopping agent.

In 2005, a British medical journal, the Lancet, raised an alarming prospect. It said dying inmates may experience searing pain from the heart-stopping chemical while lying paralyzed on the gurney if prison officials fail to give the proper dose of sodium thiopental, the anesthetic, better known as sodium pentothal.

Defense lawyers and death penalty opponents seized on this study and cited it as a reason to stop executions throughout the U.S. Their lawsuits revealed that state officials, instead of studying the effectiveness of the three-chemical combination, had relied on the fact that other states had adopted this approach.