DONALD TRUMP’S primary victories Tuesday present the Republican Party with a stark choice. Should leaders unite behind Mr. Trump, who has collected the most delegates but may reach the convention in July without a nominating majority? Or should they do everything they can to deny him the nomination? On a political level, this may be a dilemma. As a moral question, it is straightforward. The mission of any responsible Republican should be to block a Trump nomination and election. We do not take this position because we believe Mr. Trump is perilously wrong on the issues, although he is. His...