Lyndon Johnson simply was exasperated. Barack Obama¡¯s mischief was methodical.

Four days before the 1966 congressional elections, Johnson, asked about criticism from Richard Nixon, testily responded: ¡°I do not want to get into a debate ... with a chronic campaigner like Mr. Nixon.¡± Johnson¡¯s disparagement endeared Nixon to Republican voters, thereby propelling him toward the presidency.

Four days before Saturday¡¯s South Carolina primary, Obama improved Donald Trump¡¯s standing with Republicans by volubly deploring him and cannily placing him in the Republican mainstream: ¡°He says in more interesting ways what the other [Republican] candidates are saying.¡± Not exactly.