President Donald Trump has enlisted the celebrity lawyer Alan Dershowitz to join his defense team in a politically charged case to secure his acquittal in the Senate impeachment trial. Mr. Dershowitz, a Harvard Law School professor emeritus, who became famous as a defense counsel for high-profile defendants like O.J. Simpson, Claus von Bülow and Mike Tyson, is expected to have only a limited role, presenting oral arguments at the Senate trial “to address the Constitutional arguments against impeachment and removal,” the legal team said in a statement.