James K. Polk

After building a law practice in Tennessee, Polk rose through Jackson's Democratic machine: state legislature in 1823, Congress in 1825, Speaker of the House by 1835 — the only person to hold that gavel and later the presidency. He won the Tennessee governorship in 1839 but lost twice more, making him a dark horse when the deadlocked 1844 convention reached for a compromise candidate. He narrowly beat Henry Clay, pledged a single term, and set to work. Texas joined the Union his first year, triggering the Mexican–American War; the settlement handed the U.S. the entire Southwest and a border at…