WASHINGTON – The Supreme Court’s unity under new Chief Justice John Roberts was shattered yesterday by a dispute over an old adage: A man’s home is his castle.

The justices ruled 5-3 that police without a warrant cannot search a house when one resident agrees but another says no.

Justice Roberts wrote his first dissent, a harsh complaint that police may now be helpless to protect domestic abuse victims.

The decision ended a trend of one-sided rulings by the court. About two thirds of the 30 decisions under Justice Roberts’s leadership have been unanimous, a high number on a court that has in the past been polarized along ideological lines.

The court’s liberal members, joined by centrist Anthony Kennedy, said yesterday that an officer responding to a domestic dispute call did not have the authority to enter and search the home of a small-town Georgia lawyer in 2001 even though the man’s wife invited him in.

Janet Randolph called police to the home in Americus, Ga., and – over her husband’s objections – led the officer to evidence used to charge Scott Randolph with cocaine possession. That charge has been on hold while courts considered whether the search was constitutional.

The state of Georgia had the backing of the Bush administration and 21 other states that argued cooperation with law officers should be encouraged.

The case turned on the Constitution’s ban on unreasonable searches – with a twist. Justices looked at the rights of people who share their homes, a common situation in America where many households include extended families.

“The law acknowledges that although we might not expect our friends and family to admit the government into common areas, sharing space entails risk,” Justice Roberts wrote in a dissent that hinted that he lost votes in the 4 1/2 months it took the justices to resolve the case.

Justice Souter, the court’s only unmarried member, saw it differently.

“There is no common understanding that one co-tenant generally has a right or authority to prevail over the express wishes of another, whether the issue is the color of the curtains or invitations to outsiders,” Justice Souter wrote for the majority.

In all, the eight members who participated in the case wrote six different opinions, swapping barbs. Conservative Justice Antonin Scalia and liberal Justice Stevens disputed whether the ruling helped women.

Justice Stevens said: Neither husband nor wife “is a master possessing the power to override the other’s constitutional right to deny entry to their castle.”

Justice Scalia said: “I must express grave doubt that today’s decision deserves Justice Stevens’ celebration as part of the forward march of women’s equality.”

It was surprising, considering that the court in recent months has been harmonious on emotional issues including abortion limits, religious freedom, and a protest of the military’s “don’t ask, don’t tell” policy on gays.

“I’m thunderstruck this case would have generated this kind of response,” said Kermit Hall, a historian and president of the State University of New York at Albany. “My sense is we are going to see more difference than we’re going to see agreement.”