Earlier this month, almost a year from the day when Barack Obama rode the wave of history into Grant Park, he had one of those weeks that makes his presidency seem, at times, so confounding. First Obama endured an electoral embarrassment, watching his party lose off-year gubernatorial elections in New Jersey and Virginia, in part because many of the voters he had so successfully engaged in his presidential campaign, particularly younger voters, stayed home and made popcorn for “Dancing With the Stars” instead. The president had been exposed, the pundits solemnly declared, as having no “coattails,” which are something you really need, they said, if you want congressmen and senators to go along with you on hard votes. Then, just five days...