After last November's defeat, Democrats were like aviation investigators sifting through twisted metal in a cornfield, struggling to posit theories about the disaster all around them. Some put the onus on John Kerry, saying he had never found an easily discernable message. Others, including Kerry himself, wrote off the defeat to the unshakable realities of wartime, when voters were supposedly less inclined to jettison a sitting president. Liberal activists blamed mushy centrists. Mushy centrists blamed Michael Moore. As the weeks passed, however, at Washington dinner parties and in public post-mortems, one explanation took hold not just among Washington insiders but...