After leaving congress in 1839, Polk attempted to become the governor of Tennessee but lost the re-election. Sadly he lost running for governorship also in 1843. Polk was the first dark horse presidential candidate which meant he wasn't known very well but unexpectedly wins. In 1844 Polk was an unexpected democratic presidential candidate and won due to his running mate Martin Van Buren's failure to secure his party nominations and his opposition for the annexation. Polk won the election for democratic president while George M. Dallas as his vice president. During the official election Polk ran against U.S. senator Henry Clay and his campaign group that he founded called the Whigs created a slogan that stated "Who is James K Polk?" which stood for Polk's lack of acknowledgement in the world of politics. Thinking their slogan would support their campaign, the Whigs didn't expect Polk's favoring of the annexation would catch the voters attention. Polk ended up winning by a forty nine percent popular vote.
President Polk took his oath at the U.S. capitol on March 4, 1845 and made his plan for his four year term. his four point plan made up of expansion of the U.S. which includes New Mexico, Oregon and California. Also the reduction of tariffs and the revival of the Independent Treasury system. Finally the issue of slavery in the states that were soon to be obtained. Another thing that was accomplished during Polk's presidency was the U.S. Naval academy which was appointed to the Secretary of the Navy, George Bancroft and also the rushing of Irish immigrants due to the potato famine from 1845 to 1846. .
Former president John Tyler had already approved the annexation of Texas before Polk was president. Although Texas was independent from Mexico, Mexico did not like the annexation and threatened war if the annexation was approved thus inheriting a war for Polk when he entered office. On December 29, 1846 Texas became the twenty eighth state in the U.