Earlier quoted context omitted.
That's true. J Edgar Hoover did do this. Of course, Hoover is also one of the great villains of US history, and there was a major course-correction in the 1970s regarding the legal process federal law enforcement has to adhere to. "The FBI is run by the modern equivalent of J. Edgar Hoover" is also an amusing and dramatic interpretation of the facts we have now. But they don't square well with other facts we know, su…
Hoover had plenty of apologists who were arguing that he was using these tactics only for good causes (such as against communism, today it would be terrorism or child pornography), it took a long time before anybody dared to speak out against this, he was simply too powerful. Most likely today such direct blackmail of (domestic) politicians and other officials is not the case but you can't really rule it out complete…
Not only that but when someone's life is as public as a politician - and their career dependent on that credibility - it takes far less than Hoover-era style blackmail to ruin such a person.
So even if the FBI is no longer acting like the Hoover era agents there are significant careers risks in standing up to the three-letter agencies, (and their similarily-powerful network of friends and former employees). Those risks have only grown as the power of the NSA/FBI have grown with the spread of technology/internet.
The bar for making such a claim is much lower than in the 1950s, especially in the context of a politician.