James Polk went to the 1844 Democratic Convention supporting the presidential front-runner Van Buren. Polk wanted to be the vice president but became the presidential nominee by accident.
Van Buren lost Southern support because he opposed the annexation of Texas, so he gave up his delegates (who vote at the convention for their choice of nominee) to Polk, who supported it. Andrew Jackson used his influence to suggest that a Democratic nominee be chosen who reflected the nation's "manifest destiny": its inevitable westward expansion. (When something is "inevitable," is means it can't be avoided or prevented.)
Polk won the nomination. The opposition Whigs and their nominee, Henry Clay, began to ask scornfully, "Who is Polk?" How did Polk and the Democrats respond?