Sentence:  And it is clearly argued by Lord Percy of Newcastle , in his remarkable long essay , The Heresy of Democracy , and in a more general way by Voegelin , in his New Science of Politics , that this same Rousseauan idea , descending through European democracy , <WSD>is</WSD> the source of Marx 's theory of the dictatorship of the proletariat . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)