1909 United States Senate election in Wisconsin

The 1909 United States Senate election in Wisconsin was held in the 49th Wisconsin Legislature between January 27, 1909, and March 4, 1909. Incumbent Republican U.S. senator Isaac Stephenson was ultimately re-elected on the 23rd ballot after more than a month of voting and negotiation.

In the 1909 term, Republicans held overwhelming majorities in both chambers of the Wisconsin Legislature, so had more than enough votes to elect a Republican United States senator. However, this was the first U.S. Senate election in Wisconsin after the passage of the state law which established primary elections for determining party nominees. This led to considerable turmoil in the legislative joint session, as the winner of the Republican primary (Stephenson) lacked majority support in the Republican caucus and received only 31% of the primary vote. The issue was exacerbated by allegations that Stephenson had committed campaign finance violations during the primary.

Both chambers initially voted on January 26 in sufficient numbers to re-elect Stephenson, but at the joint session on January 27, the presiding officer, Lieutenant Governor John Strange, ignored a motion to read the results and pronounce Stephenson elected, and instead moved to a new vote for U.S. senator. Stephenson fell short in that vote, and the stalemate dragged out for more than a month as various factions attempted to coalesce around an alternative. March 4, 1909—the scheduled start of the next U.S. Senate term—was the effective deadline for the Legislature to act, as Stephenson already planned to assert his right to be seated as U.S. senator based on the initial January 26 votes. With the Legislature unable to reach a majority for any alternative candidate, Stephenson was ultimately re-elected by a bare majority on March 4, with more than 40 members casting protest votes for random colleagues and neighbors.