The encyclopedia Britannica claims:
Hoover habitually used the FBI’s enormous surveillance and information-gathering powers to collect damaging information on politicians throughout the country, and he kept the most scurrilous data under his own personal control. He used his possession of these secret files to maintain himself as the FBI’s director and was apparently able to intimidate even sitting presidents by threatening to leak damaging disclosures about them.
How good is the evidence for the claim that Hoover repeatidly used the intelligence gathered by the FBI to blackmail politicians to stay in power?