Jackson lobbied for the annexation of Texas, insisting that it belonged to the United States as part of the Louisiana Purchase. He thought that annexation would cause national division over slavery, but feared the British could use Texas as a base to threaten the United States. Jackson wrote several letters to Texas president Sam Houston, urging him to wait for the Senate to approve annexation and explaining how much Texas would benefit as a part of the United States. Tyler signed a treaty of annexation in April 1844, but it became associated with the expansion of slavery and was not ratified. Henry Clay, the Whig nominee for the 1844 presidential election, and Van Buren, Jackson's preferred candidate for the Democratic Party, both opposed annexation. Disappointed by Van Buren, Jackson convinced Polk, who was to be Van Buren's running mate, to run as the Democratic Party's presidential nominee instead. Polk defeated Van Buren for the nomination, and Jackson convinced Tyler not to run as an independent by bringing him back into the Democratic Party. Polk won the election, the Senate passed a bill to annex Texas, and it was signed on March 1, 1845.
