President Donald Trump on Thursday threatened Elon Musk’s federal contracts, a remarkable escalation in a public feud between the president and the world’s richest man, his former ally.

“The easiest way to save money in our Budget, Billions and Billions of Dollars, is to terminate Elon’s Governmental Subsidies and Contracts,” Trump wrote on his social media platform Thursday afternoon. “I was always surprised that Biden didn’t do it!”

The president’s relationship with Musk has deteriorated rapidly since Musk left the White House last week. The acrimony went public when Musk publicly slammed Trump’s sweeping domestic policy package on Tuesday.

He’s continued to lash out at the White House in the days since — with Musk baiting Trump by name earlier Thursday, and Trump responding by chastising the Tesla CEO from the Oval Office later in the day.

Still, Trump’s criticism from the White House — where the two men less than a week ago shared a laudatory sendoff for Musk — was not as pointed as the president’s barbs on social media.

“Without me, Trump would have lost the election, Dems would control the House and the Republicans would be 51-49 in the Senate.,” Musk wrote on X Thursday, the social media site he owns. “Such ingratitude.”

Musk’s companies have significant ties to the federal government, even before the Trump administration. SpaceX is one of NASA’s largest contractors. And his car company Tesla benefitted from a clean energy subsidy that is on the chopping block in Republicans’ reconciliation package.

“Elon was ‘wearing thin,’ I asked him to leave, I took away his EV Mandate that forced everyone to buy Electric Cars that nobody else wanted (that he knew for months I was going to do!), and he just went CRAZY!” Trump posted.

“Such an obvious lie. So sad,” Musk fired back.

Trump has previously boosted Tesla because of his close relationship with Musk. In March, with the company’s stock at a low after public anger over job cuts fueled by Musk’s DOGE initiative, the president toured different Tesla models at a makeshift car show on the White House lawn. Trump later purchased his own Tesla, “a show of confidence and support” for Musk.

Trump has routinely wielded the power of the executive branch against institutions that he deems are misbehaving. He’s frozen billions in federal grants to some of the country’s top universities, Harvard chief among them, as punishment for alleged antisemitism and civil rights violations. And he’s secured multimillion dollar deals with law firms weary of his threats to tank their business.