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Democratic nomination

Despite his loss, Polk was determined to become the next vice president of the United States, seeing it as a path to the presidency. Van Buren was the frontrunner for the 1844 Democratic nomination, and Polk engaged in a careful campaign to become his running mate. The former president faced opposition from Southerners who feared his views on slavery, while his handling of the Panic of 1837—he had refused to rescind the Specie Circular—aroused opposition from some in the West (today's Midwestern United States) who believed his hard money policies had hurt their section of the country. Many Southerners backed Calhoun's candidacy, Westerners rallied around Senator Lewis Cass of Michigan, and former Vice President Johnson also maintained a strong following among Democrats. Jackson assured Van Buren by letter that Polk in his campaigns for governor had "fought the battle well and fought it alone". Polk hoped to gain Van Buren's support, hinting in a letter that a Van Buren/Polk ticket could carry Tennessee, but found him unconvinced.

The biggest political issue in the United States at that time was territorial expansion. The Republic of Texas had successfully revolted against Mexico in 1836. With the republic largely populated by American emigres, those on both sides of the Sabine River border between the U.S. and Texas deemed it inevitable that Texas would join the United States, but this would anger Mexico, which considered Texas a breakaway province, and threatened war if the United States annexed it. Jackson, as president, had recognized Texas independence, but the initial momentum toward annexation had stalled. Britain was seeking to expand her influence in Texas: Britain had abolished slavery, and if Texas did the same, it would provide a western haven for runaways to match one in the North. A Texas not in the United States would also stand in the way of what was deemed America's Manifest Destiny to overspread the continent.

Clay was nominated for president by acclamation at the April 1844 Whig National Convention, with New Jersey's Theodore Frelinghuysen his running mate. A Kentucky slaveholder at a time when opponents of Texas annexation argued that it would give slavery more room to spread, Clay sought a nuanced position on the issue. Jackson, who strongly supported a Van Buren/Polk ticket, was delighted when Clay issued a letter for publication in the newspapers opposing Texas annexation, only to be devastated when he learned Van Buren had done the same thing. Van Buren did this because he feared losing his base of support in the Northeast, but his supporters in the old Southwest were stunned at his action. Polk, on the other hand, had written a pro-annexation letter that had been published four days before Van Buren's. Jackson wrote sadly to Van Buren that no candidate who opposed annexation could be elected, and decided Polk was the best person to head the ticket. Jackson met with Polk at the Hermitage on May 13, 1844, and explained to his visitor that only an expansionist from the South or Southwest could be elected—and, in his view, Polk had the best chance. Polk was at first startled, calling the plan "utterly abortive", but he agreed to accept it. Polk immediately wrote to instruct his lieutenants at the convention to work for his nomination as president.

Despite Jackson's quiet efforts on his behalf, Polk was skeptical that he could win. Nevertheless, because of the opposition to Van Buren by expansionists in the West and South, Polk's key lieutenant at the 1844 Democratic National Convention in Baltimore, Gideon Johnson Pillow, believed Polk could emerge as a compromise candidate. Publicly, Polk, who remained in Columbia during the convention, professed full support for Van Buren's candidacy and was believed to be seeking the vice presidency. Polk was one of the few major Democrats to have declared for the annexation of Texas.

The convention opened on May 27, 1844. A crucial question was whether the nominee needed two-thirds of the delegate vote, as had been the case at previous Democratic conventions, or merely a majority. A vote for two-thirds would doom Van Buren's candidacy due to opposition from southern delegates. With the support of the Southern states, the two-thirds rule was passed. Van Buren won a majority on the first presidential ballot but failed to win the necessary two-thirds, and his support slowly faded. Cass, Johnson, Calhoun and James Buchanan also received votes on the first ballot, and Cass took the lead on the fifth. After seven ballots, the convention remained deadlocked: Cass could not reach two-thirds, and Van Buren's supporters became discouraged about his chances. Delegates were ready to consider a new candidate who might break the stalemate.

When the convention adjourned after the seventh ballot, Pillow, who had been waiting for an opportunity to press Polk's name, conferred with George Bancroft of Massachusetts, a politician and historian and longtime Polk correspondent, who had planned to nominate Polk for vice president. Bancroft had supported Van Buren's candidacy and was willing to see New York Senator Silas Wright head the ticket, but as a Van Buren loyalist, Wright would not consent. Pillow and Bancroft decided if Polk were nominated for president, Wright might accept the second spot. Before the eighth ballot, former Attorney General Benjamin F. Butler, head of the New York delegation, read a pre-written letter from Van Buren to be used if he could not be nominated, withdrawing in Wright's favor. But Wright (who was in Washington) had also entrusted a pre-written letter to a supporter, in which he refused to be considered as a presidential candidate, and stated in the letter that he agreed with Van Buren's position on Texas. Had Wright's letter not been read he most likely would have been nominated, but without him, Butler began to rally Van Buren supporters for Polk as the best possible candidate, and Bancroft placed Polk's name before the convention. On the eighth ballot, Polk received only 44 votes to Cass's 114 and Van Buren's 104, but the deadlock showed signs of breaking. Butler formally withdrew Van Buren's name, many delegations declared for the Tennessean, and on the ninth ballot, Polk received 233 ballots to Cass's 29, making him the Democratic nominee for president. The nomination was then made unanimous.

The convention then considered the vice-presidential nomination. Butler advocated for Wright, and the convention agreed, with only four Georgia delegates dissenting. Word of Wright's nomination was sent to him in Washington via telegraph. Having declined by proxy an almost certain presidential nomination, Wright also refused the vice-presidential nomination. Senator Robert J. Walker of Mississippi, a close Polk ally, then suggested former senator George M. Dallas of Pennsylvania. Dallas was acceptable enough to all factions and gained the nomination on the third ballot. The delegates passed a platform and adjourned on May 30.

Many contemporary politicians, including Pillow and Bancroft, later claimed credit for getting Polk the nomination, but Walter R. Borneman felt that most of the credit was due to Jackson and Polk, "the two who had done the most were back in Tennessee, one an aging icon ensconced at the Hermitage and the other a shrewd lifelong politician waiting expectantly in Columbia". Whigs mocked Polk with the chant "Who is James K. Polk?", affecting never to have heard of him. Though he had experience as Speaker of the House and Governor of Tennessee, all previous presidents had served as vice president, Secretary of State, or as a high-ranking general. Polk has been described as the first " dark horse " presidential nominee, although his nomination was less of a surprise than that of future nominees such as Franklin Pierce or Warren G. Harding. Despite his party's gibes, Clay recognized that Polk could unite the Democrats.