The 1918 United States Senate election in Mississippi was held on November 3, 1918. Incumbent Democratic U.S. Senator James K. Vardaman ran for re-election to a second term in office, but was defeated in the Democratic primary by U.S. Representative Pat Harrison.
Because Harrison faced only nominal opposition in the general election, his victory in the August 20 primary was tantamount to election.
Background
In 1911, former Governor of Mississippi James K. Vardaman challenged and defeated Senator LeRoy Percy in the Democratic Party primary election, the first in state history. The victory established Vardaman and Theodore Bilbo as the uncontested leaders of the state Democratic Party. Vardaman established a reputation as a populist opponent of the state's planter elite and a white supremacist critic of the Theodore Roosevelt administration on racial issues.