Karl Marx, Friedrich Engels. The Communist Manifesto (№ 232691)

... in those periods when the class struggle nears, the process of decomposition within the ruling class, inside of the old society takes such a violent, such a dramatic character that a small section of the ruling class renounces it and joins the revolutionary class, the class that holds the future. That's why, as before, part of the nobility passed to the bourgeoisie, so now a portion of the bourgeoisie goes over to the proletariat, it is part of the bourgeois ideologists, who have risen to the theoretical understanding of the historical movement.