Billionaire presidential adviser Elon Musk attacked White House trade counselor Peter Navarro as “dumber than a sack of bricks” as a fight over President Donald Trump’s reciprocal tariff regime spilled onto social media on Tuesday.

In a flurry of posts on his social media platform X, Musk called Navarro “truly a moron” and mocked the trade adviser for in the past citing the work of “Ron Vara” — a fictional expert whose name is an anagram of his own.

The missives were the latest volley in an increasingly bitter spat between the two that broke out after Trump last week announced that he was imposing a minimum 10% tariff on most imports, with additional levies on around 60 countries that have larger trade deficits with the US. The announcement was seen as a policy win for Navarro, who has advocated aggressive trade measures in hopes of helping to boost domestic manufacturing.

Shortly after the announcement, Musk jabbed at Navarro by saying his degree in economics from Harvard is “a bad thing, not a good thing” and criticizing the trade adviser for not having built companies on his own.

Navarro, in a subsequent interview with CNBC, said the criticism was not surprising from the Tesla Inc. CEO because he was a “car assembler” who used parts from other countries.

“When it comes to tariffs and trade, we all understand in the White House — and the American people understand — that Elon is a car manufacturer, but he’s not a car manufacturer. He’s a car assembler,” Navarro said.

“He’s a car person. That’s what he does, and he wants the cheap foreign parts,” he added.

Those claims earned Musk’s ire; in addition to the personal attacks, Musk defended Tesla as “the most vertically integrated auto manufacturer in America with the highest percentage of US content.”

The dispute within the ranks of senior Trump aides comes as many companies, nations, and trade groups have expressed frustration over the uncertainty around how to best negotiate a reduction to Trump’s tariffs. The higher duties on select countries are set to go into effect after midnight.

The president has said that while he is open to making deals for the right concessions, he is seeking relief from both other countries’ tariffs and non-tariff barriers, like regulations and currency manipulation.