Honoring his pledge to serve only one term , Polk declined to seek re-election . At the 1848 Democratic National Convention , Cass led on every ballot , though it was not until the fourth that he attained a two-thirds vote . William Butler , who had replaced Winfield Scott as the commanding general in Mexico City , won the vice presidential nomination . The 1848 Whig National Convention nominated Taylor for president and former congressman Millard Fillmore of New York for vice president
1848 Democratic National Convention
1848 Democratic National Convention
James K. Polk