James K. Polk(1795-1849). Texas history resource provides an analysis of Polk's impact on the very existence of the state.
From the site:
James K. Polk never set foot in Texas. His impact on Texas and the American West, however, cannot be overestimated. Not only was he instrumental in the annexation of Texas, but the United States achieved its greatest territorial expansion under his presidency.
Born 2 November 1795 in North Carolina, Polk spent much of his youth in central Tennessee. Trained as a lawyer, Polk's interest in politics surfaced at an early age. He was elected to the Tennessee House of Representatives in 1823.
Fellow Tennessean Andrew "Old Hickory" Jackson took such an interest in the young statesman that for years Polk was referred to as "Young Hickory." Under "Old Hickory's" tutelage, Polk served seven consecutive terms in the U. S. House of Representatives. Upon Jackson's urging, Polk ran and was elected Governor of Tennessee in 1839. He then became a darkhorse candidate for president in 1844.