When he was in politics, Horner was very well-known for being a right-wing politician. He strongly supported the Progressive Conservative Prime Minister from 1957 to 1963, John Diefenbaker. Many of his family were very important in Western Canadian politics. They were all part of a group called "Diefenbaker's cowboys". Horner tried to become the leader of the Progressive Conservative Party many times. After he lost to Joe Clark in trying to become the leader in 1976, he crossed the floor, or changed the political party he was part of. He went from the Progressive Conservative Party to the Liberal Party.