President Donald Trump assailed Elon Musk on Sunday night, describing him as “off the rails” after Musk said he was creating a new political party amid an ongoing rift with the president.

“I am saddened to watch Elon Musk go completely ‘off the rails,’ essentially becoming a TRAIN WRECK over the past five weeks,” Trump wrote on Truth Social on Sunday evening. “He even wants to start a Third Political Party, despite the fact that they have never succeeded in the United States.”

Musk’s effort to create a new political party, called the America Party, is the latest rupture in his relationship with Trump. Musk, the world’s wealthiest man, spent hundreds of millions of dollars to support Trump’s presidential campaign, and then Trump rewarded him with a wide-ranging position overseeing drastic cuts to government staffing and contracts.

Their close bond crumbled in a public spectacle last month, as Trump pushed his sprawling domestic policy bill through Congress. Musk panned the legislation, which is projected to add trillions to the federal debt, as a “disgusting abomination.” He has said he would support primary challengers against any Republican who voted for the legislation, which passed with almost unanimous Republican support, but he has given few details about his new political party.

“Backing a candidate for president is not out of the question, but the focus for the next 12 months is on the House and the Senate,” Musk wrote on his social platform X on Sunday.

Trump has also threatened to cut billions of dollars in federal contracts and tax subsidies for Musk’s companies.

Trump said Sunday night that Musk had opposed the legislation because it eliminated the electric vehicle mandate, which would have been a boon for Tesla, one of Musk’s companies.

“I have campaigned on this for two years and, quite honestly, when Elon gave me his total and unquestioned Endorsement, I asked him whether or not he knew that I was going to terminate the EV Mandate — It was in every speech I made, and in every conversation I had,” Trump wrote in his post. “He said he had no problems with that — I was very surprised!”

Musk did previously support ending the electric vehicle tax credits, but has done an about-face more recently, as Tesla’s sales have dropped this year.