Well, there are two…
Well, there are two major possibilities as to the kind of information Felt might have had on hand.
Haldeman was referring to dark, nefarious things that Felt could reveal, which we now know about, but that we didn’t know about around October 1972.
Haldeman was referring to dark, nefarious things that Felt could reveal, at least some of which we still don’t know about.
There may be no way to find out, of course. But it’s worth noting that Felt was sitting on top of millions of pages of records relating to COINTELPRO at the time. We found out about COINTELPRO a year before (1971), but the knowledge of its history and operations was pretty limited in 1972 (systematic investigations didn’t begin until 1976). If Felt decided to retaliate against the Nixon administration by releasing several volumes of uncensored records of the details of government surveillance, deception, and malfeasance, at a time when the Watergate scandal was already heating up and the Pentagon Papers a recent memory, then I don’t doubt it would have been pretty hot news at the time, and catastrophic for an administration that was already in deep, deep trouble.
Alternatively, it might have been a matter of old J. Edgar’s blackmail files. Maybe he had dirt (personal or otherwise) that Nixon was afraid of; maybe not. Nixon had his own share of personal demons; but would stuff that just made him look venal or ridiculous be that much worse than what was already coming out about him?
Of course, it could be something else entirely.
Anticopyright.