Sam Giancana
At one point, Chicago mob boss Sam Giancana and his men wanted to kill Sinatra. Apparently, they tasked Sinatra with getting JFK to stop the crackdown on organized crime. An FBI member had wiretapped a member of the mob and recorded him saying, "Let's hit Sinatra. Or I could whack out a couple of those other guys." The two ended up with a mutually beneficial deal.