1918 United States Senate elections

The 1918 United States Senate elections were held throughout 1918, the midpoint of Woodrow Wilson's second term as president. This was the first election since the enactment of the Seventeenth Amendment that all 32 Class 2 senators were subject to direct or popular election, making them the final class under the old system of being selected by state legislatures. Special elections were also held to fill vacancies.

Republicans gained a slim 2-seat control after picking up a net 6 seats. This came after an April 1918 special election where they flipped a seat in Wisconsin.

Retirements
Three Republicans and one Democrat retired instead of seeking re-election. Two Republicans retired instead of seeking election to finish the unexpired terms, one Democrat retired instead of seeking election to a full term and two Democrats retired instead of seeking election to finish the unexpired terms.

Defeats
Eight Democrats and one Republican sought re-election but lost in the primary or general election.

Deaths
One Democrat died on October 21, 1917, and his seat remained vacant until an April 1918 election.

Post-election changes
Source: United States Senate Official Website

Special elections during the 65th Congress
In these special elections, the winner was seated during 1918 or before March 4, 1919; ordered by election date.

Elections leading to the 66th Congress
In these general elections, the winners were elected for the term beginning March 4, 1919; ordered by state.

All of the elections involved the Class 2 seats.

Closest races
Eighteen races had a margin of victory under 10%:

Idaho (special)
Incumbent Democratic Senator John Frost Nugent defeated Republican nominee Frank Robert Gooding by a very narrow margin of 1.00% and by 970 votes. Upon his election, John Frost Nugent became the first Democrat ever to have been elected to the United States Senate in Idaho or from Idaho and the first non-Republican to win a United States Senate seat in Idaho or from Idaho since 1901 and the first non-Republican to win the Class 3 Senate seat in Idaho or from Idaho since 1897.