BROWNSVILLE, Tex. — Shirley Menard says she did not think all that much about Donald J. Trump’s proposed border wall when she cast a reluctant vote for him in 2016.

She is thinking about it a lot more now that plans call for part of what could be a 30-foot-tall stretch of wall to land in her backyard, dividing the River Bend Resort and Golf Club , which hugs the Rio Grande. About 70 percent of the community — some 200 properties — would be stranded south of the barricade but north of the river. Fifteen holes of the golf course could be there, too.

An unsettling lesson in unexpected consequences has left some residents rethinking their support for the wall and the president who has made it his signature project.

“I never thought they’d go through a subdivision,” said Ms. Menard, a former Houston schoolteacher who said she had been shaken since she was notified in June of plans to build the wall next year. “My blood pressure has not been normal since I got that letter.”