times he was away from college, Steinbeck worked as a farm laborer, sometimes living with migrants in the farm’s bunkhouse. 

Steinbeck endeavored to speak out on behalf of migrant workers while working for as a reporter. The story was...

...based on newspaper articles about migrant agricultural workers that he had written in San Francisco.

Steinbeck's desire was to help the migrant workers who had been displaced from their farms in the Midwest by natural disaster: drought dried up the land and crops; the soil became so loose that wind storms created what became known as the Dust Bowl. Banks forced small farms to stop working: houses were bulldozed and the farmers—some who had worked the same land for generations—were forced (with physical threats and those of the law) to leave their homes, where parents had been buried, babies born and families raised.

Ads for workers ultimately drew three hundred thousand people to California looking for work during the disastrous economic times of the Great Depression of the 1930s. However, flyers (in the novel) asked for five hundred people (for example), for work needing only two hundred. Eventually, perhaps a thousand people would come for a job that required only two hundred workers. When this happened, powerful farmers dropped their wages so that workers could not even feed their families; the work was completed in half the time, cutting short the jobs parents had hoped would last longer and support their families.

When the "Okies" (a term of contempt coined for those from Oklahoma—though the Dust Bowl moved to other states as well) arrived, they were berated, cheated, turned away, and even arrested and jailed if they complained that the pay offered for a job was too low. Then they were called "reds" and were often beaten by sheriffs and deputies, until they became "bull-simple," either pretending to be simple-minded to avoid further trouble, or having suffered brain damage.

Steinbeck may have seemed a political activist in criticizing the small transient shantytowns, each called "Hooverville:"

They were named after the President of the United States at the time, Herbert Hoover, because he allegedly let the nation slide into depression.

If people in Hooverville did not cooperate when contractors (who often conducted their searches for workers illegally) arrived to hire people, Steinbeck described what would happen. Character Tom Joad at one point says he would speak out; if necessary, he would fight anyone who would stand in his way to find honest, fairly-compensated work to support his family:

I ain't gonna take it...I'll kick the hell outa somebody.

Floyd, a man from this particular shantytown explained:

You're nuts...They'll pick you right off. You got no name, no property. They'll find you in a ditch...Be one little line in the paper—know what it'll say? 'Vagrant found dead.' An' that's all.

If more than one person complained, the sheriff would decide that the Department of Health had condemned the site, evict everyone and burn it all out.

Steinbeck became an advocate of the disenfranchised. He may have seemed to have a political agenda in criticizing how badly these people were treated. 

Steinbeck was a supporter of the rights of the individual, the "little guy" and he commiserated with the farmers who were the big losers in the Dust Bowl phenomenon of the 1930s.

Ultimately, Steinbeck probably wrote The Grapes of Wrath to promote social consciousness and awareness of the plight of the poor, migrants who were displaced during the Depression.

John Steinbeck also embraced the philosophy of the Transcendentalists, especially Emerson who propounded the concept of the "Oversoul."  While men are, indeed, individuals, they do need to work with each other.  In Chapter 8 of his seminal work, "The Grapes of Wrath," Steinbeck's character Jim Casey expresses this idea of men in harmony with nature, working together for the good of all; doing so is something holy and accomplishes much.

Is it possible to work together in society and still live individual lives? Of course! Steinbeck did not devalue the idea of individualism; he valued and respected the individual. What he rejected was an economic system in which those with money and power exploited and abused those without means and influence. This was the same philosophy that once drove emigrants from the Old World into the American colonies. (Read Crevecoeur's "Letters from an American Farmer.")

When The Grapes of Wrath was published, it laid bare the abuse of economic power as Steinbeck observed it, personally. Naturally, those who wielded that economic power (especially in Oklahoma and California where the book was banned) were outraged because their practices had been challenged. Often, when working men and women seek to form a union to even the odds and better their working conditions, those in power cry "Socialism!" In Steinbeck's time, they cried "Communism!" Discussions of socialism and communism, of course, serve to divert attention from the real issues.

Steinbeck believed in the individual and the individual American Dream. Look at George and Lennie in Of Mice and Men. They did not dream of working in a commune, side by side with others; they wanted a piece of land that belonged to them where they could live their individual lives in peace and security.

John Steinbeck grew up in the area around Salinas, California. Although his parents were not poor, Steinbeck witnessed many of the injustices forced upon migrant farm workers who were coming from states like Oklahoma. These migrant worker were former sharecroppers who had lost their farms in the Dust Bowl of the 1930's. When published in 1939, the book was severely criticized for being a "socialist" or "communist" novel. This was because the government camp depicted in the novel and the idea that people should work together for the common good, was thought of as a communist idea. The novel was even criticized on the floor on Congress. However, in order to research the novel, Steinbeck traveled with some of the "Oakies" and saw the poor treatment they were given in many towns where their labor was vital to the agricultural industry. Today, many of Steinbeck's "socialist" ideas do not seem radical at all. The idea that men need to work together for the common good is receiving even more attention now that the world's economic situation and global warming pose a danger to all of mankind. Instead of being a labeled a radical socialist, Steinbeck is now thought of as someone whose ideas should be recognized as foreseeing solutions to even larger problems.