(CNSNews.com) - Hundreds of former commanders and military colleagues of presumptive Democratic nominee John Kerry are set to declare in a signed letter that he is "unfit to be commander-in-chief." They will do so at a press conference in Washington on Tuesday. "What is going to happen on Tuesday is an event that is really historical in dimension," John O'Neill, a Vietnam veteran who served in the Navy as a PCF (Patrol Craft Fast) boat commander, told CNSNews.com. The event, which is expected to draw about 25 of the letter-signers, is being organized by a newly formed group called Swift Boat Veterans for Truth."We have 19 of 23 officers who served with [Kerry]. We have every commanding officer he ever had in Vietnam. They all signed a letter that says he is unfit to be commander-in-chief," O'Neill said.