In 1844, former president Martin Van Buren attempted to win the Democratic nomination for president but lost support due to his belief that Texas should not be annexed. At the beginning of the convention, Polk was only being considered as a vice presidential candidate. However, the delegates were having a difficult time getting the necessary two-thirds-majority vote to nominate a candidate. Polk was nominated as the candidate on the ninth ballot. Because of this, he is considered to be the first dark horse candidate for president. A dark horse is a person who no one expected to be a viable candidate to win an election.

Polk was opposed in the election by Whig candidate Henry Clay. The campaign itself centered on the annexation of Texas, which Polk supported and Clay opposed. Due to Polk's support and popularity, the annexation passed three days before the end of John Tyler's term in office. For the election itself, each candidate received 50 percent of the popular vote, but Polk managed to win 170 out of 275 electoral votes.