WILLIAM, a tractor-trailer driver, leans against his navy blue truck and stares across the highway to the crowded hills of Tijuana. He voted for Donald Trump, but roundly opposes the president’s plans for a new border wall. “People will find a way around any wall. And it’s going to be you and me paying for it,” he warns, as NPR, a public-radio station, streams from his radio. His attitude, which is surprisingly widespread along the border, hints at trouble for the president.

On April 10th the customs and border protection agency announced that it will test prototypes of Donald Trump’s proposed wall somewhere in the Otay Mesa area after it chooses finalists this summer. A barricade covered in solar panels, a wall topped by a monorail and an obstacle course in which one of the barriers is a 100-foot ditch full of nuclear waste are just some of the hundreds of proposals the department will choose from.

Otay Mesa is a natural place to test a wall (which is unlikely to be so whimsical), suggests Eric Frost, who directs the homeland-security graduate programme at San Diego State University. The border crossing is one of the county’s busiest but Otay Mesa still has enough open land for new construction. “The prototypes need to interact with real people and real cars and real trucks. It doesn’t make sense to build them in the middle of the desert,” he says.

The trouble is that Otay Mesa is Democrat territory: the congressional district in which it lies voted overwhelmingly for Hillary Clinton. Nationally, only 8% of Democrats support a new border wall, compared with 74% of Republicans, according to the Pew Research Centre, a think-tank. Moreover, Republicans who live close to Mexico seem less keen on a wall than Republicans farther from the border. Locals are used to the sight of immigrants working at undesirable jobs, explains Silvia Lopez, a registered Republican who sells insurance in an Otay Mesa strip mall. Like other border-dwellers, she knows something that other Americans often forget: “We already have a wall.”

California’s lieutenant-governor, Gavin Newsom, has hinted that California’s powerful environmental laws could stymie building. Obstruction could spread. Of the 22 counties in California, Arizona, New Mexico and Texas that share a border with Mexico, 14 voted for Mrs Clinton last November (see map).

At least the federal government owns a lot of land in southernmost California. In Arizona, a large stretch of the border belongs to the Tohono O’odham, a Native American tribe. In Texas, which has far less fencing than America’s other border states, most border land is in state or private hands. The Trump administration would have to use eminent domain to acquire any private lands—a lengthy process that involves negotiations, value appraisals and, often, lawsuits. Time, perhaps, for Mr Trump to demonstrate his vaunted deal-making skills.