Joaquin Castro

The 14-year-old Caleb Gámez-Cuéllar and his parents and younger brother sat in the first line of benches in a makeshift courtroom at the Dilley Immigration Processing Center on Monday, waiting anxiously to meet the member of Congress they hoped would help get them out.

Representative Joaquin Castro of Texas, had driven 75 minutes on a bus with a group of his Democratic colleagues, from San Antonio to this desolate stretch of South Texas, with the express purpose of meeting this particular family and making the case for their immediate release.

It was his latest trip to the Dilley center, a sprawling prison fashioned out of trailers, which serves as the country’s largest family immigration detention site. Shuttered by President Joseph R. Biden Jr. in 2024 but reopened by President Trump last year, the site has become a symbol of the indiscriminate nature of the administration’s crackdown. And Mr. Castro, a seven-term Democrat from San Antonio, is on a crusade to close it.

In the case of the Gámez-Cuéllar family, their story did most of the work for him. Before landing at Dilley, Caleb and his older brother, Antonio, had been celebrated mariachi musicians from McAllen, invited by their Republican congresswoman last summer to perform at the Capitol and then visit the White House. Antonio had been crowned the best trumpeter in Texas.

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