William L. Barney, "Breckinridge, John Cabell," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00142.html.
A friend and confidant of both President Franklin Pierce and the Illinois senator Stephen Douglas, Breckinridge was one of the congressional intermediaries who convinced Pierce to accept southern demands that Douglas's bill for organizing the Kansas and Nebraska territories include the statement that the Missouri Compromise restriction on slavery was "inoperative and void." Breckinridge also played a key role in securing approval of Douglas's bill in the House. Until this point he had been viewed as a moderate from the Border South on the slavery issue. Hereafter, although Breckinridge still tried to be a sectional mediator, he was associated with the extreme demands of the Lower South. When the Kansas-Nebraska Act passed in 1854, it triggered a storm of protest in the North that spawned the Republican party. Although he sponsored little in the way of major legislation, Breckinridge was a popular congressman whose charm and affability won him many friends. This popularity, combined with his representation of a key border state that traditionally had voted for the Whigs in presidential elections, gained him the Democratic nomination for vice president in 1856. The Buchanan-Breckinridge ticket carried Kentucky and the election. Breckinridge's service as vice president, however, was even less distinguished than his congressional record. He was virtually ignored by James Buchanan and shut out of the administration's policy decisions. It was with some relief that Breckinridge looked forward to entering the U.S. Senate, having been elected in 1859 by the Kentucky legislature to fill the seat to be vacated by John J. Crittenden in 1861.