The large Mexican Cession of land divided the United States politically , with many in the North insisting that slavery not be allowed there ( and offering the Wilmot Proviso to ensure it ) , while others wanted slavery barred north of the Missouri Compromise line of 36 ° 30 ′ N. Both proposals were anathema to many Southerners , and the controversy split the Democrats . At the 1848 Democratic National Convention , the majority nominated former Michigan senator Lewis Cass for president , while a minority broke off to become the Free Soil Party , backing former president Van Buren . The Whigs chose General Zachary Taylor , a Louisianan , whose views on most political issues were unknown . Despite his past support for Van Buren , Pierce supported Cass , turning down the quiet offer of second place on the Free Soil ticket , and was so effective that Taylor , who was elected president , was held in New Hampshire to his lowest percentage in any state
1848 Democratic National Convention
1848 Democratic National Convention
Franklin Pierce