The 1875 United States Senate election in Wisconsin was held in the 28th Wisconsin Legislature between , and Incumbent Republican U.S. senator Matthew H. Carpenter ran for a second six-year term, but was defeated by a determined rebellion from within his own party. Former state legislator, Republican Angus Cameron, was elected on the 11th ballot. This was the first time that one of Wisconsin's U.S. senators had been defeated seeking re-election.
At the start of the 1875 term, Republicans held majorities in both chambers of the Wisconsin Legislature with 14 votes to spare, so should have been able to quickly elect a Republican But accusations of corruption against the incumbent, Matt Carpenter, had caused a significant number of Republican legislators to pledge to voters that they would not support Carpenter's re-election to the U.S. Senate. Despite Carpenter winning enough support in the Republican caucus to obtain renomination, the holdouts refused to fall in line with the party and denied his re-election. Ultimately, Cameron was selected as a consensus choice merging the support of the 19 holdout Republicans with 49 of the 52 Democratic, Liberal Republican, or Reformer legislators.
Prior to the caucus, Carpenter faced opposition in the caucus, but was still widely expected to win re-election in Wisconsin newspapers. During the two months between the legislative elections and the start of the legislative session, Carpenter was consumed with activity in Washington, D.C., where he was serving as presiding officer of the Senate due to the absence of Vice President Henry Wilson. At the time, Carpenter's leading opponent for the nomination was former Wisconsin governor Cadwallader C. Washburn, who had been a candidate for U.S. senate in each of the last four opportunities. The Wisconsin press concluded that one of the two would certainly be the next senator.
The Republican caucus met on the evening of January 21. On the first ballot, Carpenter secured the nomination with 54 of 81 votes. Republican newspapers urged legislators to fall in line and support the nominee, as rumors began to emerge of a planned revolt.
The legislature met in joint session on January 27 with only one member absent. They took two votes in succession to try to pick a U.S. senator. The results were identical, no candidate reached a majority.
The legislature reconvened in joint session on January 28 and took three more votes. The three votes had nearly identical results; again no candidate reached a majority.
Two more votes were taken on January 29, with identical results.
A larger number of legislators were absent from the votes on January 30 and February 1, reducing the number needed for a majority, but still no candidate could reach the threshold.
On the evening of February 2, 1875, the Democrats caucused with the holdout Republicans and agreed to a compromise candidate to merge their voting power—the caucus chose former speaker Angus Cameron. Wisconsin newspapers credited U.S. representative and railroad financier Alexander Mitchell with facilitating the negotiation. The legislature re-convened in joint session on February 3, and voted again, with nearly all the Democrats supporting Cameron, he achieving a majority on the 11th overall ballot.