With Mexico still refusing to pay for President Donald Trump’s southern border wall, the president has a new idea: solar panels.

In a meeting Tuesday with congressional Republicans, Mr. Trump suggested putting solar panels on the wall to help the project pay for itself, according to two lawmakers who attended.

Mr. Trump has insisted that the wall, a core campaign promise, will be built to prevent illegal immigration and drug smuggling, and that Mexico will pay for it. After Mexican leaders pushed back strongly against the idea, the president-elect backtracked in January, suggesting the U.S. would pay upfront for the barrier and Mexico would return the money later. But Congress hasn’t allocated any money for the project, and opposition from Democratic and some Republican lawmakers has left the wall’s future in doubt. It remains unclear whether there will be a border wall at all or how many miles of additional fencing might be built.

A White House spokesman didn’t immediately offer a comment.

The details of the plan weren’t clear, but revenue from selling the solar power generated by the panels could defray construction costs.