Victoria Reggie Kennedy, widow of Senator Edward M. Kennedy, has invited Senator Scott Brown and Democrat Elizabeth Warren to a debate in September moderated by Tom Brokaw. The debate would be co-sponsored by the Edward M. Kennedy Institute for the United States Senate and the University of Massachusetts Boston. “The missions of the EMK Institute and the university dovetail perfectly with the goal of a serious Senate debate and exchange of ideas,” Vicki Kennedy wrote. It would be Sept. 26.