During the 2008 Democratic primaries, then-Sen. Barack Obama caused a stir when he said, "Ronald Reagan changed the trajectory of America in a way that, you know, Richard Nixon did not, and in a way that Bill Clinton did not." Pundits speculated whether the optimistic, charismatic, Obama could move the nation's politics to the left in a way comparable to how President Reagan had moved the country to the right. This comparison took hold early in the Obama presidency as his approval ratings nose-dived in a pattern similar to Reagan's. The problem now for Democrats is that, though both presidents...