Throughout his life, Steinbeck was sympathetic to the plight of the working class in the United States. Most of his novels, for example, are about the struggles of everyday people to survive in a system that works against them. He was born and grew up in an agricultural valley near the Pacific Coast (Salinas), and many of his themes and characters came from his intimate knowledge of the migrant and blue-collar workers who populated the Salinas and Monterrey Bay area.
In Tortilla Flat (1955), for example, Steinbeck created a wonderfully diverse picture of life in essentially a slum area near Monterrey, CA, populated by characters who were at the edges of society and who really didn't care whether they succeeded or not in terms of middle-class material success. His characters were outside the mainstream of American life, and Steinbeck was one of the first American writers to concern himself almost exclusively with people who were not part of the middle or upper classes but whose lives had completely redeeming value.
Arguably Steinbeck's best novel, The Grapes of Wrath (1939), came out of Steinbeck's own experience when he joined an itinerant group moving from Oklahoma to California and spent months working alongside these people in the fields of California. This experience results in a novel that is intensely sympathetic to the working class and deeply suspicious of the land-owning class. Among other things, Steinbeck strenuously advocates the creation of labor unions to protect the rights of migrant agricultural workers. In his much later novel Cannery Row (1955), which, in some respects is reminiscent of Tortilla Flat, Steinbeck focuses on characters just after WWII, who have come home disillusioned, in some cases, traumatized, and characters who essentially reject the middle-class values of the 1950s and live a marginal, but free, life in and around Monterrey.
Steinbeck made a trip to the Soviet Union in the late 1940s and seems to have gotten on the FBI's "watch list" at that point. At one point, Steinbeck wrote a letter to a government official asking him to call off the FBI's surveillance because they were becoming too obtrusive. This visit, however, did not appear to result in Steinbeck becoming a huge fan of communism, but there is no doubt that Steinbeck's sympathies were with the working class, and he looked on anything that bettered the lives of working-class people as a good thing.
Steinbeck's political belief system, which was certainly to the Left, did not prevent him from supporting the United States in Vietnam. As a war correspondent, Steinbeck wrote several very pro-government pieces about our involvement in the war and, in fact, he was criticized by the anti-war faction for his support of the war.
While Steinbeck's political views were to the left of center (closer to socialism that to communism), I think it's correct to say that his politics were a reflection of his powerful and life-long support of people who were disadvantaged because they were not at the center of American society. Further, Steinbeck's politics were a result of this empathy for the common man rather than the cause.