Big John Tanner is one of the most influential politicians in the Nevada legislature.
When Roger, as politician Burt Jarvis, allows Big John Tanner to tear up his bill for clean water in "Kiss Kiss, Cam Cam" while inviting him over to fish at his lake house, he explains to Steve that he doesn't have the political clout to stand up to him. He contrasts Big John's influence against that of Little John Tanner, who complains that he is representing the good people of Winnemucca until Roger kicks him out of sight.
Roger later discovers that another of his personas is the CEO of Tetradual Inc., the company that is causing the pollution, so he has himself shut down the facility and complete his campaign promise to fix the water problem, however, he refuses to run for reelection, and his CEO becomes a campaign contributor to Big John, getting the invitation to the lake house as he ponders the reason why he is known as "Big John."
Big John is voiced by Sam Elliott.