https://en.wikipedia.org/wiki/J._Edgar_Hoover
For the FBI to spy on elected officials and public figures, and use their secrets to blackmail them into doing what the FBI wants, is more than a realistic concern: it's a historical fact that government power-brokers ran that kind of racket on a large scale, spanning decades. Hoover's technical capabilities were a fraction of the digital era's. These are incredibly dangerous powers to have concentrated in one place.
I just finished reading The Devil's Chessboard about Allen Dulles and the nascent CIA. It's a great book if you haven't read it.
The uni-party keeps them all rich. They are corrupt politicians who know the corrupt FBI people very well. Both keep each other in check as a kind of mutually assured destruction.
This means they can keep the money and power pipeline to themselves by using that to kick out anyone who does the wrong thing. Want to argue about gender or abortion? They fundamentally don't care either way. Want to deal with insider trading or stop our wars? You get your dirt dumped and the entire machine turns against you.