The Heart is a Lonely Hunter
Jake and Copeland express their dreams for the future through Marxist ideas, highlighting two disenfranchised groups: the nation's poor laborers (like Jake) and individuals in racial minority groups (like Copeland). The social injustices that Jake and Copeland experience make them extremely angry, and their Marxist ideas often veer into somewhat violent territory:
"The bastards who own these mills are millionaires. [...] So when you walk around the streets and think about it and see hungry, worn-out people and ricket-legged younguns, don't it make you mad? Don't it?"
Jake's face was flushed and dark and his lips trembled. The three men looked at him warily. Then the man in the straw hat began to laugh. (2.4.116-17)
So, while Marxism helps to sum up huge segments of American society during the Great Depression, these ideas also represent the outsider nature of Jake and Copeland and the struggles and injustices that define both men. Heck, Copeland named one of his sons Karl Marx. Now that's devotion to a cause.