The 1958 United States Senate election in Montana took place on November 4, 1958. Incumbent United States Senator
1952, ran for re-election. Mansfield won the Democratic primary comfortably, and moved on to the general election, where he was opposed by Lou W. Welch, a millworker and the Republican
nominee. In contrast to the close campaign in 1952, Mansfield defeated Welch in a landslide and won his second term in the Senate easily.