(By Larry J. Sabato, politics professor and director of U.Va.’s Center for Politics) If there is one nightmare common to all U.S. senators, it’s the possibility of an unexpected upset by an underdog challenger come Election Day. Not only do they lose their seat, but the shock of defeat becomes one of the most notable parts of their biography. This November, no one wants to be the Senate’s Eric Cantor. For my money, one of the most jaw-dropping Senate results in modern history occurred exactly 50 years ago.