Here is how Cohn threatened him for Hoover:
Later, Gallagher, then in Congress, was approached by McCarthy’s sidekick, Roy Cohn, who, by that time, was one of Hoover’s political hit men, with a huge FBI dossier on then-Presidential candidate Bobby Kennedy. When Gallagher refused to hold hearings on the FBI’s charges against RFK, mafioso Sid Zagri, who accompanied Cohn, offered him a $100,000 bribe—and Cohn added a threat, that Hoover would not consider Gallagher a “friend,” if he refused. “I’m everybody’s friend, Roy,” the Congressman replied, “but I’m nobody’s whore.” Soon after that, Cohn returned with another demand from Hoover, which Gallagher rejected. Cohn threatened, “This is the last chance you are going to get!”
The Congressman’s next encounter with Cohn involved even cruder threats from Hoover: “You’ll be sorry! Because if you’re not their friend, you’re Mr. Hoover’s enemy." It was right after that, that “the whole goddamned thing started,” Gallagher said, meaning the FBI’s witch-hunt against him. In August 1967, just weeks after Gallagher refused to blackmail Bobby Kennedy, Life magazine published an article tying Gallagher closely with the mob.
[source : Profile in Courage: Congressman Neil Gallagher, EIR, http://www.larouchepub.com/eiw/public/2013/eirv40n38-20130927/50-53_4038.pdf, 27th September 2013]