In 1957, the Cleveland police showed up at Dollree Mapp’s home looking for a bombing suspect. Ms. Mapp would not let them in without a search warrant, but they entered anyway. The police did not find the bomber, but they came across a trunk containing “lewd and lascivious” books and pictures.

Ms. Mapp was convicted of possessing obscene materials, even though the evidence was taken without a warrant. She was tried in state court, like the overwhelming majority of criminal defendants. So it did her no good that federal courts had applied the so-called “exclusionary rule” since 1914 to bar the use of illegally seized evidence.

In 1961, in Mapp v. Ohio, the Supreme Court reversed Ms. Mapp’s conviction and adopted the exclusionary rule as a national standard. The court acknowledged that the rule might let some criminals go free, but it underscored that it was more important to compel the nation’s police forces to obey the law.

The court carved out exceptions over the years, but the basic rule laid down in Mapp has endured for nearly five decades. Now, Chief Justice John Roberts’s conservative majority on the Supreme Court is working to undo the exclusionary rule in a more fundamental way. It’s been a longstanding interest of Mr. Roberts’s. As a young Reagan administration lawyer, he worked on what he described in a memo as a “campaign to amend or abolish” the rule.