There is a deep and long lived relation between the Chicago RAT machine and the mob. It even had its own Congressional seat for years filled by Frank Annunnzio. Giancana's son-in-law was his chief of staff.
Most importantly what they learned was that Gus Alex, who had come into the organization as a bodyguard to Jake Guzak, was the outfit's controller over Congressman William L. Dawson. Alex and Dawson were so close, in fact, that Alex had a gambling pallor set up across the street from Dawson's home district office.
It was due to their control over Dawson that the outfit was able to control the Black Ghettos that they flooded with drugs, crap games, whorehouses and loan sharks.
Dawson was the absolute ruler over all the Black second ward and recognized spokesman for Chicago's black population. In Washington Dawson headed up the powerful House Committee on Expenditures and in Chicago he was known as Cook Counties Democratic machine patronage dispenser.