The Supreme Court ruled on Monday that former President Donald J. Trump is entitled to substantial immunity from prosecution, delivering a major statement on the scope of presidential power. The ruling will almost surely delay the trial of the case against him on charges of plotting to subvert the 2020 election past the coming election in November.

The vote was 6 to 3, dividing along partisan lines.

Chief Justice John G. Roberts Jr., writing for the majority, said Mr. Trump had at least presumptive immunity for his official acts. He added that the trial judge must undertake an intensive factual review to separate official and unofficial conduct and to assess whether prosecutors can overcome the presumption protecting Mr. Trump for his official conduct.

That will entail significant delays, and the prospects for a trial before the election seem vanishingly remote. If Mr. Trump prevails at the polls, he could order the Justice Department to drop the charges.

The decision was one of the most important decisions on the presidency in the history of the court. The majority said protecting all presidents from having to second-guess their actions for fear of possible prosecution was a vital constitutional command.

The dissenting justices said the majority had created a kind of king not answerable to the law.

Broad immunity for official conduct is needed, the chief justice wrote, to protect “an energetic, independent executive.”

“The president therefore may not be prosecuted for exercising his core constitutional powers, and he is entitled, at a minimum, to a presumptive immunity from prosecution for all his official acts,” Chief Justice Roberts wrote. “That immunity applies equally to all occupants of the Oval Office, regardless of politics, policy or party.”