RICHMOND, Va.—A top Republican Virginia lawmaker was forced to defend his role editing a college yearbook with photographs of students in blackface, as the state’s political crisis spread to both parties Thursday. Senate Majority Leader Thomas Norment served as managing editor of the 1968 yearbook at Virginia Military Institute that included the blackface photos and racial slurs. Gov. Ralph Northam, Democrat, faces calls to step down following the emergence last week of a photo from his medical-school yearbook that showed a person in blackface and another in Ku Klux Klan garb. Mr. Northam said he wasn’t in the picture, after...