OF THE many unrealistic promises Donald Trump made during his campaign—restoring lost manufacturing jobs, immediately deportating 2m to 3m undocumented immigrants—building a wall that entirely divides America from Mexico was always among the least plausible. First there is the question of cost, which the Massachusetts Institute of Technology, a university, estimates could be anywhere between $27bn and $40bn depending on the structure’s height, length and depth under the ground. There is the issue of geography. The ever-shifting sands of the Algodones Dunes in south-eastern California, for example, make a fixed wall nearly impossible. And there are legal questions, which Xavier Becerra (pictured), the attorney-general of California, evidently has a lot of.

On September 20th Mr Becerra, who has sued the Trump administration eight times in recent months, announced that his state would again challenge the administration in court, this time to halt construction of Mr Trump’s proposed border wall and prototypes of that border wall. The 53-page lawsuit he filed makes 11 separate complaints. Among them is the point that Mr Trump’s wall would violate the National Environmental Policy Act (NEPA), which the suit calls the “basic national charter for protection of the environment,” because the administration failed to assess its potential effect on wildlife, fish and water. Another is that the wall’s construction would violate the Tenth Amendment, which enshrines the principle of states’ rights. And it argues that the threat of the wall has already had and will continue to have a “chilling effect” on tourism from Mexico to California, thus costing the state dear. A report by Tourism Economics, a global research firm, estimates Mexican tourism to the Golden State will fall 7% in 2017, and continue to drop in 2018. When your correspondent visited an area of the border near San Diego in the spring, taxi drivers who wait by the port of entry to ferry Mexican shoppers to nearby outlet malls expressed similar fears.

The lawsuit comes a few weeks after the Department of Homeland Security, the federal body which oversees immigration and border security, declared it had awarded contracts to four companies to build prototypes of concrete walls in Otay Mesa, a busy border area near San Diego. Mr Becerra announced his legal challenge not far from there at Border Field State Park in front of the existing border fencing—a tall chain-link fence and tightly-spaced metal stakes that protrude into the Pacific Ocean—as if to make the point that Mr Trump’s wall would not only be illegal but an unnecessary waste of money. Alternating between English and Spanish, Mr Becerra bellowed: “The border between the U.S. and Mexico spans some 2,000 miles. The list of laws violated by the president’s administration in order to build his campaign wall is almost as long.”

In some ways, California should have been the easiest place for Mr Trump to build his border wall. Most of the land along the border there is owned by the federal government. The same is not true in other border states. 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 land along the border is in state or private hands. The Trump administration would have to exercise eminent domain to acquire any private lands—a lengthy and controversial process that involves negotiations, value appraisals and, often, lawsuits. Even if Mr Trump prevails against California in court, the path to building his wall seems longer and rockier than the border he hopes to secure.