WASHINGTON—For the second time this century, the U.S. Supreme Court finds itself playing perhaps its least comfortable role: ruling on a presidential election. 

Former President Donald Trump, the front-runner for the Republican nomination, was barred from the Colorado ballot after state courts found he engaged in a failed insurrection following his 2020 defeat. He has promised an appeal that may force the nine justices to decide the meaning of 14th Amendment provisions originally adopted to keep ex-Confederates from taking power in the government they once sought to violently overthrow.