He benefited from the fact that he had been out of the country as ambassador in London and had not been involved in slavery issues. Buchanan and running mate John C. Breckinridge of Kentucky carried every slave state except Maryland, defeating anti-slavery Republican John C. Frémont and Know-Nothing former president Millard Fillmore to win the 1856 presidential election.
As President, Buchanan intervened to assure the Supreme Court's majority ruling in the pro-slavery decision in the Dred Scott case. He acceded to Southern attempts to engineer Kansas’ entry into the Union as a slave state under the Lecompton Constitution, and angered not only Republicans but also Northern Democrats. Buchanan honored his pledge to serve only one term and supported Breckinridge's unsuccessful candidacy in the 1860 presidential election. He failed to reconcile the fractured Democratic party amid the grudge against Stephen Douglas, leading to the election of Republican and former Congressman Abraham Lincoln.
Buchanan's leadership during his lame duck period, before the American Civil War, has been widely criticized. He simultaneously angered the North by not stopping secession and the South by not yielding to their demands. He supported the ineffective Corwin Amendment in an effort to reconcile the country. 