Though the justices offered different reasons, the decision was unanimous. All agreed that individual states may not bar candidates for federal office under a constitutional provision, Section 3 of the 14th Amendment, that bars insurrectionists from holding office, New York Times reported.

The decision was the court’s most important ruling concerning a presidential election since Bush v. Gore handed the presidency to George W. Bush in 2000.

The case arose from a challenge brought by six Colorado voters who sought to disqualify Mr. Trump from the ballot for the state’s Republican primary based on Section 3 of the 14th Amendment. The provision was adopted after the Civil War to forbid those who had taken an oath “to support the Constitution of the United States” from holding office if they then “shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof.”

A Colorado trial judge ruled that Mr. Trump had engaged in insurrection but accepted his argument that Section 3 did not apply to the president or to the office of the presidency.

The Colorado Supreme Court affirmed the first part of the ruling — that Mr. Trump had engaged in an insurrection. Among his efforts, as detailed in the courts’ opinions: setting out to overturn the result of the 2020 presidential election; trying to alter vote counts; encouraging bogus slates of competing electors; pressuring the vice president to violate the Constitution; and calling for his supporters to march on the Capitol.

But the Colorado Supreme Court’s majority reversed the part of the trial judge’s decision that said Section 3 did not apply to the president or the presidency.