President Trump is grappling with the gravest controversy of his presidency as the storm kicked up by his firing of FBI Director James Comey rumbles on. An atmosphere of crisis enveloped Washington in the first full day after the firing, which was announced Tuesday. Cable news shows were thick with talk of a constitutional crisis and comparisons to the Watergate scandal of the early 1970s that brought down President Richard Nixon. On Capitol Hill, the fissures in the Republican Party over the episode were plain, with some prominent GOP lawmakers hitting Trump’s actions and others backing him up. Democrats, meanwhile,...