Differences of OpinionWhile political watchers spent last week looking ahead to primaries in Ohio and Texas, the candidates engaged in a serious debate — over a photo of Barack Obama wearing Somali clothing. (An Obama staffer claimed Hillary Clinton had leaked the shot to make him look Islamic; Clinton’s campaign manager said no one had claimed the photo was “divisive” until Obama and his new friend at the Post played it up.) Latecomer Ralph Nader, unsafe at any speed as far as most liberals are concerned, moseyed into the presidential race. Connecticut senator Christopher Dodd backed Obama; Jersey governor Jon Corzine rushed to aid the Clintons in Cleveland.