Billionaire Elon Musk escalated his attacks against White House trade adviser Peter Navarro Tuesday, calling him a “moron” and “dumber than a sack of bricks.”

Musk, an adviser to Trump on federal spending cuts, was responding to comments made by Navarro on CNBC about how Tesla makes its cars. Navarro said Musk is "a car assembler" who relies on imports of foreign-made parts to build his electric vehicles with Tesla.

The men’s juvenile feud is a proxy for more substantive divisions within President Donald Trump’s MAGA coalition.