Ms. Gabbard confirmed on Sunday that her chief of staff received a message last Tuesday from the chief of staff to Representative Debbie Wasserman Schultz, the chairwoman of the national committee, about her attendance at the debate. A day earlier, Ms. Gabbard had appeared on MSNBC and said there should be an increase beyond the current six sanctioned debates.

A person close to the committee who asked for anonymity to discuss internal discussions insisted, however, that Ms. Gabbard had not been disinvited. Instead, the person said, an aide to Ms. Wasserman Schultz expressed a desire to keep the focus on the candidates as the debate approached, rather than on a “distraction” that could divide the party, and suggested that if Ms. Gabbard could not do that, she should reconsider going.

Ms. Gabbard insisted otherwise.