In the first two advertisements of their kind, Republicans seeking an advantage in a Mississippi special election are invoking Barack Obama in arguing that a Democratic Congressional candidate is too liberal for his district. That flies in the face of what has been conventional wisdom for months among national Democratic strategists who have not publicly taken sides in the presidential contest; many privately express more hope in Obama's potential coattails than in rival Hillary Clinton's. But while Hillary Clinton and House Speaker Nancy Pelosi have made appearances in Republican advertisements before, the GOP is now taking on Obama, reflecting both...