Barack Obama has, like most in public life, made his share of gaffes--the president of Canada, 57 states, the Austrian language, E Pluribus Unum, the pronunciation of corpsman, among others. To be sure, they are stunning signs of ignorance of things that are common wisdom for most, even Harvard alums. Closet Obama supporters will seek to downplay the incident. But his Monday mis-step is huge politically and may well come to haunt and hurt him as Republican Mitt Romney rolls out the attack plan for this fall's campaign and before. Of all the GOP wannabes, Romney has been Obama's most...