He doesn’t have a personal beef with the trucker who brought the trailer here, he said. Mr. Flores has never met Mr. Miranda, but he, too, was born in Nuevo Laredo. In 1991, he moved to Brownsville and got married. He and his wife earned money by baking and selling traditional Mexican cakes outside offices. It took him a decade to get his citizenship and cost $1,000 for lawyers and fees.

It may sound strange that an immigrant like him wants to keep Mexicans off his route. But Mr. Flores sees himself as an American who earned the right to this work. One in every five people in the business of transportation is an immigrant, according to the Pew Research Center.

“Those guys just want to come here, make their money and go home,” Mr. Flores said. “I spend my money here. I bought my house. I pay my taxes.”