President Trump on Sunday issued a lengthy Truth Social post attacking former ally Elon Musk after the Tesla mogul announced he was launching a third party over opposition to Trump’s “big, beautiful bill.”

“I am saddened to watch Elon Musk go completely ‘off the rails,’ essentially becoming a TRAIN WRECK over the past five weeks,” Trump wrote Sunday afternoon.

“He even wants to start a Third Political Party, despite the fact that they have never succeeded in the United States – The System seems not designed for them,” he added. “The one thing Third Parties are good for is the creation of Complete and Total DISRUPTION & CHAOS, and we have enough of that with the Radical Left Democrats, who have lost their confidence and their minds!”

Shortly before his post, Trump addressed Musk’s newly formed “America Party” during a gaggle with reporters in front of Air Force One.

“I think it’s ridiculous to start a third party,” Trump said in response to a reporter’s question, adding later: “So he can have fun with it, but I think it’s ridiculous.”

Trump’s comments are the latest indication that the erstwhile allies’ relationship has taken another downturn. Musk on Saturday announced that he was launching the new party in response to the passage of the reconciliation package, which Musk opposed.

“By a factor of 2 to 1, you want a new political party and you shall have it!,” Musk wrote on X. “Today, the America Party is formed to give you back your freedom.”

It’s not yet clear if Musk will follow through with the party in the wake of his announcement. But the move comes the men launched into a back and forth for the second time in a matter of months as Republicans in Congress worked to pass the megabill.

Trump has repeatedly said Musk’s opposition to the spending package is due to the bill’s elimination of incentives for electric vehicles (EVs), something the president repeated in his Truth Social post on Sunday.

“It is a Great Bill but, unfortunately for Elon, it eliminates the ridiculous Electric Vehicle (EV) Mandate, which would have forced everyone to buy an Electric Car in a short period of time,” Trump wrote. “I have been strongly opposed to that from the very beginning.”

The 2022 Inflation Reduction Act expanded and extended tax credits for consumers who bought EVs. But the “big, beautiful bill,” which Trump signed into law on Friday, eliminates those tax credits.

“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 on Sunday. “He said he had no problems with that – I was very surprised!”

In addition to the formation of the “America Party,” Musk has also vowed to use his considerable fortune to back primary challengers against Republicans who voted for the bill. He has also said he will work to defend Rep. Thomas Massie (R-Ky.), who ran afoul of Trump after opposing the spending package.

Musk and Trump have had a fraught relationship over the last couple months despite the tech entrepreneur emerging as one of the president’s closest allies in last year’s election. After feuding last month over the bill, the two men seemingly reached a detente — only to once again start sniping at each other over the past week.

Some Republicans have expressed concern that Musk could distract Republicans from unifying as they look to hold onto their slim House majority in a challenging election environment next year.

“One of the most destructive behaviors that we’ve had in cycles where we’ve been unsuccessful in Senate races … are those in which we have expended massive resources in intraparty warfare,” one Republican consultant told The Hill regarding Musk’s primary threats.