The Republic wasn't democracy, but points along the political cycle he sketched were democratic (though surely Plato wasn't thinking of anything as specific as parliamentary democracy as we know it today.)

The young Marx would have said that democracy (though not anything as specific as parliamentary democracy as we know it today - more like free association, cooperation, and individual autonomy) expressed the truth of human nature, while the old Marx would say that human nature plus the path of technological development existing over our whole history implies that at a certain point something like parliamentary democracy would be inevitable (but not irreplacable.)