Three states were admitted to the Union during Polk 's presidency : Honoring his pledge to serve only one term , Polk declined to seek re-election in 1848 . With Polk out of the race , the Democratic Party remained fractured along geographic lines . Polk privately favored Lewis Cass as his successor , but resisted becoming closely involved in the election . At the 1848 Democratic National Convention , Buchanan , Cass , and Supreme Court Justice Levi Woodbury emerged as the main contenders . Cass drew support from both the North and South with his doctrine of popular sovereignty , under which each territory would decide the legal status of slavery . Cass led after the first ballot , and slowly gained support until he clinched the nomination on the fourth ballot . William Butler , who had replaced Winfield Scott as the commanding general in Mexico City , won the vice presidential nomination . Cass 's nomination alienated many northerners and southerners , each of whom saw Cass as insufficiently committed to their position on the slavery issue
1848 Democratic National Convention
1848 Democratic National Convention
Presidency of James K. Polk