Cass resigned as governor in 1831 to accept appointment as Secretary of War under Andrew Jackson. As Secretary of War, he helped implement Jackson's policy of Indian removal. After serving as ambassador to France from 1836 to 1842, he unsuccessfully sought the presidential nomination at the 1844 Democratic National Convention; a deadlock between supporters of Cass and former President Martin Van Buren ended with the nomination of James K. Polk. In 1845, the Michigan Legislature elected Cass to the Senate, where he served until 1848. Cass's nomination at the 1848 Democratic National Convention precipitated a split in the party, as Cass's advocacy for popular sovereignty alienated the anti-slavery wing of the party. Van Buren led the Free Soil Party's presidential ticket and appealed to many anti-slavery Democrats, possibly contributing to the victory of Whig nominee Zachary Taylor.