Vladislav Inozemtsev has a fascinating piece in The March/April 2012 issue of The American Interest, The Cultural Contradictions of Democracy: The main threats to democracy lie within liberal societies themselves. This is not an oversimplified "waiting room reading" piece. It will take work to follow his argument, but it is worth the work.
In essence, Inozemtsev argues that democracy works so long as (a) majorities can change, so that those who are in the minority at one point can reasonably expect to be in the majority at other points (otherwise it simply becomes the tyranny of a permanent majority), and (b) everyone sees themselves as members of a single national group. As soon as these conditions cease to hold, democracy is in trouble. So one of the things that most threatens our current democracies, according to Inozemtsev, is the liberal support of multiculturalism, which leads to groups thinking of themselves as separate "tribes" (or in the case of immigrants, helps them maintain an identity separate from the common national identity and inhibits their assimilating into the common culture). Thus instead of all of us simply being Americans, some groups have been encouraged to grow or maintain separate identities as "African-Americans" or "Hispanic-Americans" or "Asian-Americans" or.......
His arguments, as I said, take some work to follow, but his points are worth considering and thinking about. Many European nations are having serious troubles because they have been culturally resistant to assimilating immigrants from other cultures, with the result that they harbor within their boarders large "ghettos" of unassimilated foreigners who do not share the same cultural or political views as the rest of the nation. And of course there are always political leaders willing to use "multiculturalism" as a means to build and maintain their own political power base within such a separated group, and therefor to continue to encourage the culture separateness of their group.