The Whig Party split over the issue of slavery. By the 1840s, slavery was starting to become the key national issue. It permeated nearly every part of the national agenda. Whigs claimed that Southern Democrats pushed for the Mexican War in order to create a vast slavery empire. Southern Democrats claimed that the North was not committing enough infrastructure to the South, in order to hurt the slavery economy. The politicians of the day were not helpful to the Whig Party, either. The party initially formed as an opposition group to Andrew Jackson's policies and soon grew to push for a National Bank and the "American System" as seen by Henry Clay. Henry Clay should have been the winner of the presidency over the dark-horse Democrat James K. Polk of Tennessee in 1844, but Clay took different stances on slavery depending on which region of the country he was trying to appease. This cost him support nationally and Polk was elected by promising to expand the nation's borders. The Whig Party soon lost voters who, despite liking the party's platform, could not vote for either non-committal candidates on the slavery issue or candidates in favor of slavery. James Birney founded the Liberty Party in 1840 around the single issue of ending slavery. While it had no chance of winning, it allowed people to vote according to ending slavery. Finally, the Republican Party formed in the early 1850s in order to protest the expansion of slavery in the territories. This party served as the end of the Whigs, as many prominent Whigs such as Abraham Lincoln and William Seward left to become the leadership of the Republican Party.