Michael Tomasky: “Whether Obama can cut, even modestly, into Clinton’s white working-class margin is, in my view, the single most important factor that will decide who wins this nomination. If he can, he has a strong shot at Ohio (March 4) and Pennsylvania (April 22). If he can’t, Clinton will probably be able to hold him off.” In his New York Times column today, Bill Kristol argues that Obama is up to the challenge: