What is solidly grounded is this: although you'll hear a lot of bloviating about how Felt was a hero for resisting the "politicization" of the FBI under Nixon, keep in mind that Kennedy and Johnson also wanted to bring the FBI under control of elected officials, but their personal corruption meant they couldn't act against J. Edgar Hoover, who had plenty on them in his files to bring them down. After Hoover died, Nixon got up the courage to appoint someone from outside Hoover's coterie as the new Director of the FBI. But Felt, who had been Hoover's #2 and was passed over for the top job, quickly got his revenge on Nixon.
Ever since Oliver Stone's 1991 movie "JFK," conspiracy theories have been deeply out of fashion among respectable people. Yet, some fraction of history actually does consist of covert conspiracies, although the conspiracy theories that become popular (Hoover killed Kennedy) are generally less accurate than the ones that nobody cares about (Hoover's heir helped bring down Nixon). Back in early 2001, I made up a list of conspiracy theories that turned out to be, more or less, true.
The National Security State naturally generates conspiracies, and the run-up to the Iraq War will probably go down as the most fertile generator of conspiracies in American history.