Selection of Rep. Jerrold Nadler (D-N.Y.) as the ranking Democrat on the Judiciary Committee was the clearest sign yet of how seriously House Democrats consider the possibility of a full-blown constitutional showdown with Trump. Nadler anchored his candidacy for his new position, vacated with the resignation of Rep. John Conyers Jr. (D-Mich.), on the 13 years he has spent as chairman or ranking Democrat on the panel’s Constitution subcommittee and, more recently, its courts subcommittee. He also politely reminded Democrats in recent days of his efforts, beginning in the mid-1980s, to impede Trump’s efforts to develop portions of New York’s...