President Obama has come out swinging against his Republican rival, sponsoring television advertisements that ask, “What is Mitt Romney hiding?” The allusion is to such relatively minor matters as Mr. Romney’s prior tax returns, the date he stopped working for Bain Capital and the nonpublic records from his service heading the Salt Lake City Olympics and as governor of Massachusetts. Mr. Obama has defended his demands that Mr. Romney release more information about himself, declaring in August that “the American people have assumed that if you want to be president of the United