Mr. Cárdenas, the only son of a Mexican president, never won the office but became a leading figure in the country’s democratic transformation.

A rare localized peace deal announced by the first lady, Melania Trump, offered a brief respite for Kherson, where Russian drones and artillery have long terrorized civilians.

Representative Haley Stevens has been less than enthusiastic about Dr. Abdul El-Sayed, injecting late intraparty tensions into a race that could decide Senate control.