Jack Bowman is Republican Senator from Montana who is also the Chairman of the Senate Appropriations Committee. He was the Governor of Montana, but he resigned after the Capitol Bombing so his successor could appoint him to the United States Senate. During the events of the series, he continuously opposes Tom Kirkman's ideals.
Before the Capitol Bombing, Jack Bowman was governor of Montana who is a staunch conservative who is also pro-gun rights, the reason behind him being in support of gun rights is most likely because of him being from, and representing a rural state of Montana and his beliefs overall.
After the Capitol Bombing, he resigned his gubernatorial seat in order to get appointed to the U.S. Senate to fill the empty seat by his successor, his lieutenant governor.