Never mind Mexico—now the sun would pay for the wall. (Or as Tom Gleason, the firm’s founder, told E&E News: “The wall pays for itself.”)

Gleason’s proposal even included a mockup, which hints at how his firm would solve a tricky engineering problem. Solar panels usually go on roofs, not on walls, because the goal is to keep them out of shadow and expose their surface to as much sun as possible through the day. To get around this issue, Gleason angles two rows of panels slightly off the wall’s perpendicular:

Gleason Partners LLC

Gleason Partners LLC

In North America, solar panels also usually face south, toward the equator. So presumably the most expensive hardware on the wall would look toward Mexico.

From Trump, the idea seemed like a politically simplistic troll. Progressives will not magically come to support a divisive mega-project if it also subsidizes renewable firms. Environmental groups that believe the wall will hurt local ecosystems will still oppose the project even if it becomes carbon neutral. As Brett Hartl of the Center for Biodiversity said in a statement on Tuesday: “An ecological disaster with solar panels on top is still an ecological disaster. With solar panels on top.”

But it is not the first time that immigration restrictionists have borrowed environmental arguments to bolster their appeal. John Hultgren, a professor of environmental politics at Bennington College, filled a book with examples of the overlap between the two groups: the now aptly titled Border Walls Gone Green.

Some contemporary figures in immigration restrictionism began in the environmental movement. John Tanton, who founded three immigration-lobbying groups, including the Federation for American Immigration Reform, began his involvement in politics through environmental activism. He says he once lobbied the Sierra Club to adopt anti-immigration positions; when they demurred, he founded his own network of groups.

Today, the Southern Poverty Law Center calls Tanton “the racist architect of the modern anti-immigrant movement.” They cite a letter of Tanton’s held at the University of Michigan, in which he writes: “I’ve come to the point of view that for European-American society and culture to persist requires a European-American majority, and a clear one at that.” (The New York Times covered the relationship between Tanton and the SPLC in April.) Linda Chavez, a veteran of the Reagan administration, has said that Tanton is both “anti-Hispanic” and “anti-Catholic.”

Tanton’s own website describes him as a supporter of “population stabilization” and “environmentally sustainable immigration numbers.”

But the connections between pro-nature sentiment and anti-immigration politics—especially at their most racist—are strongest long before the modern era.

“Some of the earliest American environmental groups had interesting and important connections to the eugenics movement,” Hultgren told me. The most famous of these is Madison Grant, who worked to conserve huge swaths of American wilderness and helped create the national park system.