“Here in Texas, we take the concept of private property very seriously,” said Representative Henry Cuellar, a Democrat whose district includes nearly 300 miles of the border with Mexico. “We take pride in our land, which has often been passed down for generations. And Texans stand up for ourselves when the federal government tries to take what is ours.”

Ms. Garcia’s case shows how difficult seizing private land can be. Nearly a decade ago, officials from the Department of Homeland Security tried to take parts of her land in order to build a border wall. Ms. Garcia fought back in court, and this year the government decided that it didn’t need her property after all.

But now, she thinks, Mr. Trump’s plans could again imperil her land. “We’re just waiting and watching as they start talking again about building a wall,” she said.

Mr. Trump’s proposed wall would run through a vast swath of the Rio Grande Valley. In March, the Homeland Security Department issued a request for proposals to build a “physically imposing” wall on the border with Mexico. More than 100 vendors have submitted proposals, and department officials say they may notify winning contractors as early as next week. The construction of several wall prototypes is supposed to begin in San Diego this summer.

In addition, Mr. Trump wants to hire 20 lawyers to obtain land in the Southwest on which a wall or other security facilities can be built.

The Rio Grande Valley is among the busiest smuggling routes on the Mexican border. Last year, Border Patrol agents seized 326,393 pounds of marijuana, second only to the agency’s Tucson sector. It also seized about 1,460 pounds of cocaine, the most of any sector. Nearly 187,000 illegal border crossers were apprehended here in 2016, the most of any Border Patrol sector.