I'm thinking Rehnquist. Not only is he ailing and probably dieing. He is also someone who has not been suspected a trait I associate with DT. He was on the Supreme Court at that time. But he was one of the Arizona crowd. Also a friend and fellow Arizona crowd member was Kleindienst who was Attorney Gen. at the time of water gate and was convicted. They probably talked and Kleindienst probably confided with Rehnquist. i suspect that is how DT had connections with the FBI,through Kliendienst. I don't know if Kleindienst, or who, had connections with the Campaign.
Also Rehnquist had been the White house chief lawyer prior to having been appointed to the court. He could of well maintained connections to that office.
I don't know why Rehnquist would turn on Nixon (after all he appointed him to the Supreme Court.) But, Woodward said the person was motivated by conscience. That is about the best guess I can come up with. Rehnquist abstained from the Nixon case when it got to the Supreme Court. Possibly an act of conscience by someone who was more involved in the case then thought. Serving on the Supreme Court would also be a motivation by DT to keep his identity secret for the balance of his life (and his SCOTUS term). Also in only confirming the info,DT could be showing a judicial temperment in not geting to involved.
Woodward is such a fabricator and Bernstein is such a dope, I'm skeptical about the whole DT story.