The single greatest threat to democracy has always been the tyranny of the masses. It's why we have things such as the Bill of Rights and our system of checks and balances, among other things.

It's addressed in The Federalist Papers – some of my favorite collegiate reading – referring to the "the superior force of an interested and overbearing majority." (It actually comes from No. 10, penned by James Madison.)

But it's Lord Acton's reference that holds more relevance today, I think, when he wrote in 1877's The History of Freedom in Antiquity: "The one pervading evil of democracy is the tyranny of the majority, or rather of that party, not always the majority, that succeeds, by force or fraud, in carrying elections."