Biggest Upset in History?

There's a question floating around the internets about whether or not Obama's victory over Clinton is the biggest political upset in American history. Commentators who say that it is not often point to 1948's Dewey Defeats Truman. I'm not sure how one can possibly judge "bigger" or "smaller" upsets on such a timescale (after all, how accurate are our perceptions of historical contests?), but I would draw your attention to the Democratic Convention of 1896, when William Jennings Bryan stole the nomination from incumbent President Grover Cleveland.