WASHINGTON – The Supreme Court made it easier yesterday for police to barge into homes and seize evidence without knocking or waiting, a sign of the court’s new conservatism with Samuel Alito on board.

The court, on a 5-4 vote, said judges cannot throw out evidence collected by police who have search warrants but do not properly announce their arrival.

It was a significant rollback of earlier rulings protective of homeowners, even unsympathetic homeowners like Booker Hudson, who had a loaded gun next to him and cocaine rocks in his pocket when Detroit police entered his unlocked home in 1998 without knocking.

The court’s five-member conservative majority, anchored by new Chief Justice Roberts and Justice Alito, said that police blunders should not result in “a get-out-of-jail-free card” for defendants.

Dissenting justices predicted that police will now feel free to ignore previous court rulings requiring officers with search warrants to knock and announce themselves to avoid running afoul of the Constitution’s Fourth Amendment ban on unreasonable searches.

“The knock-and-announce rule is dead in the United States,” a Wayne State University professor who represented Hudson, David Moran, said. “There are going to be a lot more doors knocked down. There are going to be a lot more people terrified and humiliated.”

Supporters said the ruling will help police do their jobs.

“People who are caught red-handed with evidence of guilt have one less weapon to get off,” the legal director of the Criminal Justice Legal Foundation, Kent Scheidegger, said.

The case provides the clearest sign yet of the court without Justice Sandra Day O’Connor.

Hudson had lost his case in a Michigan appeals court. Justices agreed to hear his appeal last June, four days before Justice O’Connor’s surprise announcement that she was retiring.

Justice O’Connor was still on the bench in January when his case was first argued, and she seemed ready to vote with Hudson. “Is there no policy of protecting the home owner a little bit and the sanctity of the home from this immediate entry?” she asked.

She retired before the case was decided, and a new argument was held this spring so that Justice Alito could participate, apparently to break a 4-4 tie.

Four justices, including Justices Alito and Roberts, would have given prosecutors a more sweeping victory but did not have the vote of Justice Kennedy, a moderate conservative.

Ronald Allen, a Northwestern University Law professor, said the ruling “suggests those four would be happy to consider overturning” a 1961 Supreme Court opinion that said evidence collected in violation of the Fourth Amendment cannot be used in trials. “It would be a significant change,” he said.

Justice Kennedy joined in most of the ruling but wrote to explain that he did not support ending the knock requirement. “It bears repeating that it is a serious matter if law enforcement officers violate the sanctity of the home by ignoring the requisites of lawful entry,” he said.

Justice Kennedy said that legislatures can intervene if police officers do not “act competently and lawfully.” He also said that people whose homes are wrongly searched can file a civil rights lawsuit.

Justice Scalia, writing for the majority, said that there are public-interest law firms and attorneys who specialize in civil rights grievances.

Detroit police acknowledge violating the knock-and-announce rule when they called out their presence at Hudson’s door, failed to knock, then went inside three seconds to five seconds later. The court has endorsed longer waits, of 15 seconds to 20 seconds. Hudson was convicted of drug possession.

“Whether that preliminary misstep had occurred or not, the police would have executed the warrant they had obtained, and would have discovered the gun and drugs inside the house,” Justice Scalia wrote.

Four justices complained in the dissent that the decision erases more than 90 years of Supreme Court precedent.

“It weakens, perhaps destroys, much of the practical value of the Constitution’s knock-and-announce protection,” Justice Breyer wrote for himself and Justices Stevens, Souter, and Ginsburg.

Justice Breyer said that while police departments can be sued, there is no evidence of anyone collecting much money in such cases.