Steinbeck's greatest affinity is lavished on Jim, the communist neophyte who connects to the strikers on a metaphysical level, spiritually identifying with the mass of underprivileged humanity they represent. Jim is not perfect, yet his motives are pure. He is repeatedly aligned with ideas of mysticism/spirituality, compassion, and sacrifice.
In this book more than in any other, Steinbeck explores what he called the "group man" theory: the notion that groups are independent, "living" organisms in which the whole is greater than the sum of its parts.
Jim is the character most fully aligned with this concept, though it is Doc Burton who articulates the notion most clearly in the novel.
Outside of Jim, Steinbeck paints a largely sympathetic picture of the strikers (workers) in this novel, but does not present them as being completely moral or flawless. The growers/owners are not presented sympathetically and are seen for the most part from the perspective of the strikers.
The other communist party member in the novel, Mac, is not shown in a sympathetic light as Jim is. Mac is morally complex. He makes mistakes of calculation and emotion and sullies his character morally on several occasions.