“This non-endorsement pledge is unprecedented and is not being required of any other persons or entities,” they wrote. “To us, such a pledge seems inappropriate when a non-media sponsor issues a debate invitation. We can assure both campaigns that the debate will be fair, just as the one we cosponsored between Senator Brown and Attorney General Martha Coakley in 2010 was fair.”

Barnett responded with his two-sentence statement saying the senator would not participate in the debate.

The candidates are still slated to debate at some point on WBZ-TV, as well as before a media consortium in western Massachusetts. Warren has also accepted a debate proposed by a Boston media consortium, while Brown has accepted two Boston-area radio debate invitations.

The Brown campaign has refused to meet with the Warren staff to discuss the invitations they have solely accepted, as well as remaining invitations and the terms and dates for their possible meetings.

The Brown campaign argues that Edward M. Kennedy himself used to dictate the terms under which he would debate, and that there were no debate negotiations when Brown faced off with Coakley in 2010.

Brown has had a tumultuous relationship with the Kennedy family.

The Republican famously rebranded Edward Kennedy’s seat the “People’s Seat” - during a 2010 debate at UMass Boston - and dashed the family’s hopes that it would remain in Democratic hands after the liberal stalwart died of brain cancer in 2009.

During his 2010 campaign, Brown upset the Kennedy family by airing an ad that featured footage of President John F. Kennedy talking about the importance of tax cuts. The image of the president, the slain elder brother of Edward Kennedy, then morphed into an image of Brown, making the same argument.