(CNN) – Republican Sen. Scott Brown of Massachusetts declined an invitation from the Edward M. Kennedy Institute to debate his rival, Democrat Elizabeth Warren, citing Vicki Kennedy's plan to endorse 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's campaign manager Jim Barnett said in a statement Tuesday. "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 Kennedy Institute invited the two Massachusetts Senate candidates last week to a debate they said would be moderated by NBC News' Tom Brokaw and possibly aired on a cable television network, according to the Boston Globe.
While Warren accepted the invitation, Brown issued stipulations for his participation. In a letter to the late Sen. Kennedy's wife, Barnett wrote that Brown would only partake if Kennedy vowed not to endorse a candidate. They accepted Brokaw as moderator but said they would prefer a debate with local media sponsors, not "out-of-state cable networks with a reputation for political advocacy."
Kennedy responded to Brown through a letter Tuesday to the two campaigns from the institute.
"This non-endorsement pledge is unprecedented and is not being required of any other persons or entities," a letter read. "To us, such a pledge seems inappropriate when a non-media sponsor issues a debate invitation."
The institute's CEO and chief of staff to the office of the chancellor at the University of Massachusetts Boston assured both campaigns the debate would be fair and said further details, including the moderator and media coverage could be addressed with campaign representatives in the future.
"We very much hope that we can continue this discussion in a professional and serious manner," the letter read. "Our number one priority is to educate and engage the voters and to promote civil public discourse."
Brown, who was elected in a special election following Sen. Kennedy's death and Warren, a former consumer advocate to the Obama administration, are locked in a close election that has received national headlines. The rivals previously agreed to two other televised debates in the state.