We hear many fallacies in election years. The fallacy that seems to be most popular this year is that, if Donald Trump comes close to getting the 1,237 delegates required to become the Republican nominee, and that nomination goes instead to someone else, then the convention will have ignored "the voice of the people." Supposedly Republican voters would be outraged, many would stay home on election day, and some might even vote for the Democrats' nominee, whether Hillary Clinton or Bernie Sanders. Mr. Trump has more than once made the veiled threat that he would run as a third-party candidate...