The 1964 United States Senate election in Montana took place on November 3, 1964. Incumbent United States Senator
Mike Mansfield, who was first elected to the Senate in
1952 and was re-elected in
1958, ran for re-election. Mansfield won the
Democratic primary in a landslide, and advanced to the general election, where he faced Alex Blewett, the Majority Leader of the
Montana House of Representatives and the
Republican nominee. Though Mansfield's margin was quite reduced from 1958, he still overwhelmingly defeated Blewett and won his third term in the Senate in a landslide.