WASHINGTON — Almost every part of the Bill of Rights applies to both the federal government and to the states, but the Supreme Court on Monday wrestled with whether to tolerate a rare exception for non-unanimous jury verdicts in criminal cases. Such verdicts are forbidden in federal trials under the Sixth Amendment but permitted in ones held in state court.

There seemed little doubt that the justices would have found the question fairly easy absent a confusing 1972 decision that said the Constitution required federal juries to render unanimous verdicts but allowed divided state juries. The vote was 4 to 1 to 4, and only Justice Lewis F. Powell Jr., who cast the controlling vote, said federal and state cases could be treated differently.

Monday’s case, Ramos v. Louisiana, No. 18-5924, concerned Evangelisto Ramos, a Louisiana man who was convicted in 2016 of killing a woman in New Orleans. The jury’s vote was 10 to 2, which was enough under the state’s law at the time. Louisiana has since amended its state Constitution to bar non-unanimous verdicts, but the move came too late to help Mr. Ramos, as it applies only to crimes committed after 2018.

Oregon is the last state that allows non-unanimous verdicts in criminal cases.

Jeffrey L. Fisher, a lawyer for Mr. Ramos, urged the court to protect dissenting voices on juries. “If you have one or two members of a minority on a jury, it could be a racial minority, it could be a political minority, it could be a religious minority,” he said. “Are we really prepared to say that those one or two votes can be utterly canceled out?”