"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," said Brown campaign manager Jim Barnett 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."

Debate sponsors sent the Brown campaign a letter — obtained by the Boston Globe — in which they say Brown's request for Kennedy's widow to remain neutral as a condition for attending the debate was "inappropriate."

“Given the good will and understanding of the nonpartisan mission of the institute that Sen. 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,” the letter reads.