In a letter to the Kennedy Institute, campaign manager Jim Barnett wrote that while it was OK with the moderator, NBC's Tom Brokaw, it could not accept a network "with a reputation for political advocacy." Via The Boston Globe:

"[W]hile we accept Tom Brokaw as a moderator, we prefer debates with local media sponsors, not out-of-state cable networks with a reputation for political advocacy,” he continued. “We are confident that issue can be easily addressed as there are a number of Massachusetts media outlets that would be willing to sponsor a debate such as the one you are proposing for the Kennedy Institute, and I’m sure they would be pleased to have Mr. Brokaw as moderator."

...Sen. Brown's other caveat is that Vicki Kennedy, Ted Kennedy’s widow and the woman who invited the candidates to the debate, promise to stay neutral in the election.