OTTUMWA, Iowa—José Rodas moved to this blue-collar town on the Des Moines River in 2001 to take a job at the Cargill pork plant, after more than a decade eking out an existence in California and Nevada.

Within a year, the Guatemalan immigrant had saved enough money to buy a house for his family. Last March, he opened the town's first tortilla factory.

"It's...