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 the Pacific. He negotiated the Oregon boundary with Britain along the 49th parallel, reestablished the Independent Treasury, and slashed tariffs. He kept his promise and left in 1849. Cholera killed him in Tennessee three months later. Scholars credit him with执行力 on an agenda that made the country continental - and for widening the divide over slavery that the new territory inflamed.