On that windy September day, the trio smiled and dug shovels into a mound of dirt in front of a large sign: “Future home of SpaceX’s South Texas launchpad.”

The rural tract of land that was supposed to be home to a commercial spaceport now stands in the path of Mr. Trump’s border wall. Mr. Musk is just one of the potentially hundreds of private landowners in Texas who would be disrupted by the president’s desire to build a “big, beautiful” wall.

The border wall was a signature campaign promise for Mr. Trump, but Democrats oppose it. Congress has said it would provide up to $1.6 billion for border security this year, but Mr. Trump wants $5 billion for his wall alone — and is now threatening to shut down the government without it.