WASHINGTON On Day One as president, candidate Donald Trump promised last year, he would start working on his great wall along the southern border.

It would be 35 feet tall at least. It would be 1,000 miles long, and extend deep enough underground to prevent Mexicans from tunneling beneath it. And it would be impenetrable: Its going to be made of hardened concrete, and its going to be made of rebar. Thats steel, he explained to a Virginia audience.

Most important of all: It wouldnt cost U.S. taxpayers a dime. Whos paying for the wall? Trump would ask in a campaign rally call-and-response favorite. And his fans would answer: Mexico!

Now, it turns out, President Trumps wall may be none of those things. Early specifications call for only an 18-foot wall, although 30 feet is preferable. The material is unspecified, and the Cabinet member in charge of building it has said in some places it might be more a series of sensors than an actual structure. As to who will pay for it, it turns out Mexico is not particularly interested in doing that, leaving U.S. taxpayers on the hook if and when work actually starts. Except that Congress isnt that eager to spend tens of billions of dollars for Trumps project either.

Nobody wants to pay for it. What a shock, said Edward Alden, a research fellow at the Council on Foreign Relations and author of The Closing of the American Border . The reality is that theres not much of a constituency for the border wall.