Here's an interesting story from The Daily Beast on how Hoover handled such matters.
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Reading the Official and Confidential files that survived makes it clear they could have been gathered for no other purpose than blackmail. For example, on June 13, 1958, the head of the Washington field office informed Hoover that, prior to marrying a member of Congress, the members wife had been having an affair with a Negro [and] also at one time carried on an affair with a House Post Office employee. More recently, the report said, the congressmans wife endeavored to have an affair with [an] Indonesian, who declined.
In response to this tidbit, Hoover wrote back on June 25 that it was certainly thoughtful of you to advise me of matters of current interest, and I am glad to have the benefit of this information.
This was a way of putting congressmen on notice that we had something on them and therefore they would be more disposed to meeting the bureaus needs and keeping Hoover in power, says John J. McDermott, who headed the Washington field office and eventually became deputy associate FBI director.
Hoover let presidents know that he had dirt on them as well.
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More at link.
Just like your dentist colleague, PCIntent, these things have a way of biting back.