Beginning in 1844 Buchanan became a regular contender for his party's presidential nomination; he was finally nominated in 1856, defeating incumbent President Franklin Pierce and Senator Stephen A. Douglas at the 1856 Democratic National Convention. Buchanan and running mate John C. Breckinridge of Kentucky defeated Republican John C. Frémont and Know-Nothing former president Millard Fillmore to win the 1856 presidential election. Buchanan supported the pro-slavery decision of the Supreme Court in the Dred Scott case. He also joined with Southern leaders in attempting to admit Kansas to the Union as a slave state under the Lecompton Constitution. In the process, he angered Republicans and alienated many Northern Democrats. Buchanan held to his pledge to serve only one term and supported Breckinridge's unsuccessful candidacy in the 1860 presidential election, which was won by Republican Abraham Lincoln.