While living at the same time and holding some similar viewpoints, Hugo and Marx were not of the same ideology despite the "socialist" moniker. Their disagreement concerning the use of class-violence is notable, despite Hugo protecting communards and advocating for their amnesty following the Paris Commune, Victor chose to not participate or join the revolution due to its tactics. They were also a part of two separate political organizations, Karl Marx was a very influential member of the the International Working Men's Association while Victor had been a member of the League of Peace and Freedom. A decree was passed in 1868, in large part due to Karl Marx, officially opposing affiliation with the League. When formalizing this move, Marx notes to Frederich Engels,

You know that on the General Council I expressed my opposition to our joining the peace-at-any-price party... The main point was that these fine gentlemen from the peace congress, Victor Hugo, Garibaldi, L. Blanc, etc., had kept themselves supremely aloof from our International Association. I have now obliged them to acknowledge us as a real force.

Marx discusses Hugo more directly in the preface of his Eighteenth Brumaire of Louis Napoleon, concerning the coup of 1851:

From the above facts it will be seen that the present work took shape under the immediate pressure of events and its historical material does not extend beyond the month of February, 1852. Its republication now is due in part to the demand of the book trade, in part to the urgent requests of my friends in Germany.

Of the writings dealing with the same subject at approximately the same time as mine, only two deserve notice: Victor Hugo’s Napoleon le Petit and Proudhon’s Coup d’Etat. Victor Hugo confines himself to bitter and witty invective against the responsible producer of the coup d’etat. The event itself appears in his work like a bolt from the blue. He sees in it only the violent act of a single individual. He does not notice that he makes this individual great instead of little by ascribing to him a personal power of initiative unparalleled in world history. Proudhon, for his part, seeks to represent the coup d’etat as the result of an antecedent historical development. Inadvertently, however, his historical construction of the coup d’etat becomes a historical apologia for its hero. Thus he falls into the error of our so-called objective historians. I, on the contrary, demonstrate how the class struggle in France created circumstances and relationships that made it possible for a grotesque mediocrity to play a hero’s part.

In the many now documented letters between Karl Marx and Frederich Engels, the two express their personal grievances with nearly every popular left-wing individual at the time. Just days prior to the siege of Paris, where Hugo would turn to eating zoo animals to survive, Engels mentions Victor Hugo.

With the passing of time the war is assuming an unpleasant character. The French have not yet been thrashed enough, and the German asses have won far too many victories. Victor Hugo writes nonsense in French, and fair William [William I] abuses the German language.

Engels himself was certainly not polite to Hugo thrashing him in later letters. In one letter to Paul Lefargue, (married himself to one of Karl Marx's daughters, vocal critic of Hugo calling him "the most perfect personifications of its' [bourgeoisie's] instincts, passions and thoughts,") Engels writes,

That "unfathomable" and extremely chaotic Germany which has never existed outside Victor Hugo's imagination. The Germany which was supposed to be interested only in music, dreams and clouds, and which left the care of matters here below to the French bourgeois and journalists.