Lewis Cass (October 9, 1782 – June 17, 1866) was an American military officer and politician. During his long political career, Cass served as a governor of the Michigan Territory, an American ambassador, and a U.S. Senator representing Michigan. He was the nominee of the Democratic Party for President of the United States in 1848.
In 1820, he led an expedition to the northern part of the territory, in the northern Great Lakes region in present-day northern Minnesota, in order to map the region and discover the source of the Mississippi River. The source of the river had been unknown until then, resulting in an undefined border between the United States and British North America. The expedition erroneously identified Cass Lake as the source of the river. The source of the river was correctly identified in 1832 by Henry Schoolcraft, who had been Cass's expedition geologist, as nearby Lake Itasca.
Cass represented Michigan in the United States Senate from 1845 to 1848. He served as chairman of the Committee on Military Affairs in the 30th Congress. In 1848, he resigned from the Senate to run for President. William Orlando Butler was his running mate. Cass was a leading supporter of the Doctrine of Popular Sovereignty, which held that the people who lived in a territory should decide whether or not to permit slavery there. His nomination caused a split in the Democratic party, leading many antislavery Democrats to join the Free Soil Party. He also supported the annexation of Texas.
From 1857 to 1860, Cass served as Secretary of State under President James Buchanan. He was sympathetic to American filibusterers and was instrumental in having Commodore Hiram Paulding removed from command for his landing of marines in Nicaragua and compelling the removal of William Walker to the United States. Cass resigned on December 13, 1860, because of Buchanan's failure to protect federal interests in the South and failure to mobilize the federal military, actions that might have averted the threatened secession of Southern states.