U.S. Senator H. Alexander Smith decided not to seek re-election in 1958 and Kean became a candidate for the open seat in the United States Senate. He won the Republican Primary by 23894 votes over Bernard M. Shanley, who had served as Deputy Chief of Staff to President Dwight Eisenhower. Kean received 152,884 votes (43.00%) to 128,990 (36.28%) for Shanley. Robert J. Morris, who had served as Chief Counsel to the United States Senate Subcommittee on Internal Security, finished third with 73,658 votes (20.72%).