On June 25, 1973, former White House counsel John Dean shocked the country with his testimony to a Senate committee revealing President Richard M. Nixon’s complicity in the Watergate coverup. He recalled “telling the president that there was a cancer growing on the presidency and that if the cancer was not removed that the President himself would be killed by it.” He thereby set in motion the process that led to Nixon’s downfall a little more than a year later. It has taken 49 years, almost to the day, but on Tuesday afternoon we finally heard congressional testimony about presidential...