As Tesla shares plummeted for a fourth straight day, CEO Elon Musk let loose on President Donald Trump’s top trade advisor Peter Navarro.

Musk, the world’s richest person, started going after Navarro over the weekend, posting on X that a “PhD in econ from Harvard is a bad thing, not a good thing,” a reference to Navarro’s degree. Whatever subtlety remained at the beginning of the week has since vanished.

On Tuesday, Musk wrote that “Navarro is truly a moron,” noting that his comments about Tesla being a “car assembler,” as much are “demonstrably false.” Musk called Navarro “dumber than a sack of bricks,” before later apologizing to bricks. Musk also called Navarro “dangerously dumb.”

Musk’s attacks on Navarro represent the most public spat between members of President Trump’s inner circle since the term began in January, and show that the steep tariffs announced last week on more than 180 countries and territories don’t have universal approval in the administration.

When asked about the feud in a briefing on Tuesday, White House press secretary Karoline Leavitt said, “Look, these are obviously two individuals who have very different views on trade and on tariffs.”

“Boys will be boys, and we will let their public sparring continue,” she said.

For Musk, whose younger brother Kimbal — a restaurant owner, entrepreneur and Tesla board member — has joined in on the action, the name-calling appears to be tied to business conditions.

Tesla’s stock is down 22% in the past four trading sessions and 45% for the year. Tesla has lost more tha $585 billion in value since the calendar turned, equaling tens of billions of dollars in paper losses for Musk, who is also CEO of SpaceX and the owner of xAI and social network X.

Even before President Trump detailed his plan for widespread tariffs, he’d already placed a 25% tariff on vehicles not assembled in the U.S. Many analysts said Tesla could withstand those tariffs better than competitors because its vehicles sold in the U.S. are assembled domestically.

But the company’s production costs are poised to increase because of the tariffs on materials and parts from foreign suppliers. Canada and Mexico are among the leading sources of U.S. steel imports, and Canada is the nation’s largest supplier of aluminum, while China and Mexico are home to major suppliers of printed circuit boards to the automotive industry.

At a recent an event hosted by right-wing Italian Deputy Prime Minister Matteo Salvini, Musk said, “Both Europe and the United States should move, ideally, in my view, to a zero-tariff situation, effectively creating a free trade zone between Europe and North America.”

Musk, whose view on trade relations with Europe stands in stark contrast to the policies implemented by the president, has a vested interest in the region. Tesla has a large car factory outside of Berlin, and the European Commission previously turned to SpaceX for launches.

Even before the tariffs, Tesla’s business was faltering. Last week, the company reported a 13% year-over-year decline in first-quarter deliveries, missing analysts’ estimates. That report that landed days after Tesla’s stock price wrapped up its worst quarter since 2022.

Musk, who spent roughly $290 billion to help return Trump to the White House, is now leading the Department of Government Efficiency, or DOGE, which has slashed costs, eliminated regulations and cut tens of thousands of federal jobs. In the first quarter, Tesla was hit with waves of protests, boycotts and some criminal activity that targeted vehicles and facilities in response to Musk’s political rhetoric and his work in the White House.

WATCH: Brad Gerstner explains his Tesla position

Apple‘s 23% plunge over the past four trading sessions has again turned Microsoft into the world’s most valuable public company.

As of Tuesday’s close, Microsoft is worth $2.64 trillion, while Apple’s market cap stands at $2.59 trillion.

While the market broadly is getting hammered by President Donald Trump’s sweeping tariff plan, Apple is getting hit the hardest among tech’s megacap companies due to the iPhone maker’s reliance on China.

The Nasdaq is down 13% over the past four trading days, as President Trump’s decision to impose tariffs on imports from more than 100 countries has sparked fears of a recession brought on by rising prices. UBS analysts on Monday predicted that the price of the iPhone 16 Pro Max could jump as much as $350 in the U.S.

Both Apple and Microsoft, along with chipmaker Nvidia, were previously valued at upward of $3 trillion before the recent sell-off.

In January, Microsoft issued disappointing revenue guidance. Nevertheless, last week, as Jefferies analysts reduced their price targets on many software stocks, they wrote Microsoft was among the “companies who we view as more insulated” from tariff uncertainty.

Microsoft also had the highest market capitalization of any public company in early 2024, but Apple soon reclaimed the title.