| Election Name: | 1940 United States Senate election in Missouri |
| Country: | Missouri |
| Type: | presidential |
| Ongoing: | no |
| Previous Election: | 1934 United States Senate election in Missouri |
| Previous Year: | 1934 |
| Next Election: | 1946 United States Senate election in Missouri |
| Next Year: | 1946 |
| Election Date: | November 5, 1940 |
| Image1: | Harry S. Truman LCCN2016890263 Crop.jpg |
| Nominee1: | Harry S. Truman |
| Party1: | Democratic Party (US) |
| Popular Vote1: | 930,775 |
| Percentage1: | 51.17% |
| Nominee2: | Manvel H. Davis |
| Party2: | Republican Party (US) |
| Popular Vote2: | 886,376 |
| Percentage2: | 48.73% |
| Map Size: | 250px |
| U.S. Senator | |
| Before Election: | Harry S. Truman |
| Before Party: | Democratic Party (US) |
| After Election: | Harry S. Truman |
| After Party: | Democratic Party (US) |
The 1940 United States Senate election in Missouri was held on November 5, 1940. Incumbent Democratic U.S. Senator and future President of the United States Harry S. Truman, who was first elected in 1934, decided to seek re-election to a second term. He narrowly survived a primary challenge from Governor of Missouri Lloyd C. Stark before also narrowly defeating Republican nominee Manvel H. Davis in the general election.
Heading into the election of 1940, the Pendergast machine that had propelled Harry S. Truman to victory in the 1934 election had been brought to its knees. Tom Pendergast was ill and imprisoned for widespread voter fraud in the 1936 elections, in which more votes were tallied in Kansas City than its entire population. As such, Truman's connections to Pendergast going back to his 1922 candidacy for judge in Jackson County were a liability in the campaign. Missouri Governor Lloyd C. Stark, who had portrayed himself as an opponent of the machine, challenged Truman for the Democratic nomination for U.S. Senator.[1] [2]
Truman, who had decided to seek a second term in February 1940, overcame his Pendergast connections as well as a substantial financial deficit, defeating Stark for the Democratic nomination.[2]
Touting his experience as a World War I veteran and track record as a U.S. Senator, Truman ultimately defeated Republican candidate and former state senator Manvel H. Davis in the November general election.[2]