By Michael Levenson Globe Staff

Senator Scott Brown today rejected a debate proposed by Victoria Reggie Kennedy, after the widow of Senator Edward M. Kennedy refused his precondition that she not endorse a candidate in his reelection campaign against Democrat Elizabeth Warren.

“We respect Vicki Kennedy’s decision but we regret that we cannot accept a debate invitation from someone who plans to endorse Scott Brown’s opponent,” Brown Campaign Manager Jim Barnett said in a statement. “The Kennedy Institute cannot hold itself out as a nonpartisan debate sponsor while the president of its board of trustees gets involved in the race on behalf of one of the candidates.”

The announcement came shortly after representatives of Vicki Kennedy said she would not agree to Brown’s demand that she remain neutral in the race, in exchange for the senator’s participation in a late September debate she had proposed be hosted by the University of Massachusetts Boston and Edward M. Kennedy Institute for the United States Senate.

Barnett had said Monday that Brown would participate only if Kennedy, president of the board at the Kennedy Institute, not endorse in the race and that MSNBC not be the broadcast sponsor of the debate.

In a letter today to Brown and Warren, Lisa McBirney, chief operating officer of the Kennedy Institute, and Christopher Hogan, chief of staff in the chancellor’s office at UMass Boston, the other debate co-sponsor, made clear that Vicki Kennedy would not relinquish her right to endorse in the race.

“Given the goodwill and understanding of the nonpartisan mission of the institute that Senator Brown has thus far shown, it seems inconsistent that he would now attempt to restrict the activities of Mrs. Kennedy as a condition of accepting a debate that is co-sponsored by an organization with which she is affiliated,” McBirney and Hogan wrote.