Barack Obama’s campaign has dismissed as not believable a prominent Hillary Rodham Clinton backer’s “tortured explanation” for seeming to inject Obama’s youthful drug use into the 2008 presidential campaign, and called it “troubling” that Clinton has not done more to distance herself from the remark. Former President Bill Clinton was drawn into the controversy on Monday in appearances on black radio talk shows. He told one host, Roland Martin, that impolitic remarks by supporters sometimes “just happen” in politics and said, “I think it's important not to overreact to them.” Obama has admitted in his memoir to using drugs as...