Now that Donald Trump appears on the verge of launching a presidential campaign, it is worth reflecting on the meaning of this low moment in American political history. Trump is a clown and a buffoon, and the odds of him winning even one Republican caucus or primary appear slim. But there is no denying that Trump has managed to tap into something genuinely worrisome in American politics. Democrats may be tempted to take pleasure in the fact that Trump will likely push the GOP presidential field to the right, and thereby help Obama in 2012. But this would be sheer...