He previously was the 13th speaker of the House of Representatives (1835–1839) and ninth governor of Tennessee (1839–1841). A protégé of Andrew Jackson, he was a member of the Democratic Party and an advocate of Jacksonian democracy. Polk is chiefly known for extending the territory of the United States through the Mexican–American War; during his presidency, the United States expanded significantly with the annexation of the Republic of Texas, the Oregon Territory, and the Mexican Cession following American victory in the Mexican–American War.
After building a successful law practice in Tennessee, Polk was elected to its state legislature in 1823 and then to the United States House of Representatives in 1825, becoming a strong supporter of Andrew Jackson. After serving as chairman of the Ways and Means Committee, he became Speaker of the House in 1835, the only person to have served both as Speaker and U.S. president. Polk left Congress to run for governor of Tennessee, winning in 1839 but losing in 1841 and 1843. 