What was the dirt that Lepke Buchalter offered to Thomas Dewey so that Lepke might escape the chair?

Was it that Hoover and his #2 guy, in the FBI, Clyde Tolson, were lovers?
Was it that Hoover and Tolson were up to their necks in gambling debts owed to the mob?

Was it a combination of these tidbits?

Short answer: Lepke thought he had a "lever" because Sidney Hillman, founder of the Amalgamated Clothing Workers Union and FDR's top labor adviser ("clear it with Sidney!") paid Lepke off to the tune of $5k/week for labor peace. Longer answer follows:

Allie (Tick-Tock) Tanenbaum and Abe (Kid Twist) Reles of Murder Inc. turned rat for Brooklyn DA (later NYC Mayor) Bill O'Dwyer. They implicated Lepke in several murders. He went underground, and his massive rackets empire went to hell. Finally, he was approached by three Mob "pals"--Meyer Lanksy, Albert Anastasia and Abner (Longie) Zwillman--who convinced him to give himself up to the Feds and stand trial on a narcotis rap. They told him he'd get no more than 5-6 years, which would keep him out of Dewey's clutches and the murder rap(s). He famously met columnist Walter Winchell in Manhattan, who drove him to a car containing J. Edgar Hoover.
The advice from Lepke's three "pals" was a betrayal: Lepke got 15 years in a Federal pen. Meanwhile, Dewey, who was a power in the GOP and who was their Presidential candidate in '44 and '48, put it around that FDR had "snatched" Lepke from him in order to keep the Hillman/Lepke/FDR connection secret. So the Justice Dept. turned Lepke over to Dewey, who convicted him and sent him to the chair.
The Hoover/Tolson gay allegation was well known all around the Mob. It might have been true. But none of Hoover's many biographers has ever been able to confirm it. It is certainly true that Hoover's FBI largely ignored organized crime. But the most likely explanation is that Hoover was too busy concentrating on the Communist Party and bank robbers to pay any attention to the Mob. Hoover, an addicted horse-player, also got tips on fixed races from FBI agents with Mob connections. It's likely that Hoover knew the source of those tips.

Thanks Turnbull!

When in doubt, going to Turnbull is like going to the videotape !