President Donald Trump’s attack on solar and wind projects threatens to raise energy prices for consumers and undermine a stretched electric grid that’s already straining to meet rapidly growing demand, renewable energy executives warn.

Trump has long said wind power turbines are unattractive and endanger birds, and that solar installations take up too much land. This week, he said his administration will not approve solar and wind projects, the latest salvo in a campaign the president has waged against the renewable energy industry since taking office.

“We will not approve wind or farmer destroying Solar,” Trump posted on Truth Social Wednesday. “The days of stupidity are over in the USA!!!”

Trump’s statement this week seemed to confirm industry fears that the Interior Department will block federal permits for solar and wind projects. Interior Secretary Doug Burgum took control of all permit approvals last month in a move that the American Clean Power Association criticized as “obstruction,” calling it “unprecedented political review.”

The Interior Department blocking permits would slow the growth of the entire solar and wind industry, top executives at renewable developers Arevon, Avantus and Engie North America told CNBC.