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

In a flurry of posts on his social media platform X, Musk called Navarro, a former UC Irvine professor, “truly a moron.” He mocked the trade adviser for citing in the past 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 U.S. 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 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.”

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.”

With large plants in California and Texas, Tesla builds all of the cars it sells in the U.S. domestically. That’s in contrast to larger automakers such as Stellantis NV and General Motors Co., which import more than 40% of the vehicles sold in the U.S., according to figures from Global Data.

Between 60% and 75% of the components Tesla uses are manufactured domestically, depending on the model, according to a 2024 filing by the U.S. National Highway Traffic Safety Administration, with the majority of the remaining parts sourced from Mexico.

White House Press Secretary Karoline Leavitt downplayed the disagreement on Tuesday, telling reporters that “boys will be boys” and that the administration “will let their public sparring continue.”

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.