[WS:] Democracies have not been created equal. Arend Lijphart (_Patterns of Democracy_) distinguishes two kinds: majoritarian - prevalent in the US and other English speaking countries; and consensus - prevalent in the continental Europe and countries with proportional representation. He argues that consensus democracy is superior in representing minority interests, providing social protection, conflict resolution and violence control. I may add to this list the fact that socialist and social democratic parties do relatively well in consensus democracies had have had zero success in majoritarian ones.
As far as I can see, there is no such a thing as a majority, especially in the countries like the US, which can be best described as a plurality of minorities. Most of those minorities are not represented politically - so it should be be difficult to convince them that they have nothing to lose with the current system and they can win something with a PR system.
Wojtek