“Socialism never took root in America because the poor see themselves not as an exploited proletariat, but as temporarily embarrassed millionaires.”

As quoted in A Short History of Progress (2004) by Ronald Wright. This has since been cited as a direct quote by some, but the remark may simply be a paraphrase, as no quotation marks appear around the statement and no earlier publication of this phrasing has been located.
This is perhaps an incorrect quote from Steinbeck's article "A Primer on the '30s." Esquire, June 1960: 85-93.
"Except for the field organizers of strikes, who were pretty tough monkeys and devoted, most of the so-called Communists I met were middle-class, middle-aged people playing a game of dreams. I remember a woman in easy circumstances saying to another even more affluent: 'After the revolution even we will have more, won't we, dear?' Then there was another lover of proletarians who used to raise hell with Sunday picknickers on her property.

"I guess the trouble was that we didn't have any self-admitted proletarians. Everyone was a temporarily embarrassed capitalist. Maybe the Communists so closely questioned by the investigation committees were a danger to America, but the ones I knew—at least they claimed to be Communists—couldn't have disrupted a Sunday-school picnic. Besides they were too busy fighting among themselves."
Disputed
Source: "John Steinbeck once said that socialism never took root in America because the poor see themselves not as an exploited proletariat but as temporarily embarrassed millionaires", [Ronald, Wright, A Short History of Progress, 2004, 124, Anansi Press, Toronto, https://books.google.com/books?id=nzWPFQIEvfEC&q=%22temporarily+embarrassed+millionaires%22#v=snippet&q=%22temporarily%20embarrassed%20millionaires%22&f=false]