The establishment in this article of a Sen. Craig "hold" is based solely on the credibility of the source. The source in this case is the NY Times quoting anonymous sources.
NYTIMES: Pentagon officials express outrage that for more than a month Mr. Craig has single-handedly delayed the careers of hundreds of officers and stymied important Air Force business for a handful of parochial planes. They are vowing not to give in to his pressure. Calling the move blackmail, one senior military official said, "If we say yes to this, Katie bar the door." The official, like others contacted for this article, spoke on the condition of anonymity, fearing retribution from the senator.
Mr. Hart would say only that "Senator Craig's record of overwhelming support for the military speaks for itself" and blamed the Air Force leadership for disclosing his hold "as some sort of strategy to renege on promises made to Senator Craig."
Sounds like his spokesman has confirmed the hold to me...