The country’s biggest Republican donor called on Monday for the formation of a new political party and suggested he would back primary challengers against nearly every single Republican in Congress.

That was the saber-rattling declaration of Elon Musk, should Republicans on Capitol Hill pass President Trump’s sweeping domestic policy bill.

While Mr. Musk’s words are often just that, he has dramatically escalated his anti-Republican rhetoric over the past few days. On Monday, he suggested that if the G.O.P. bill passed, he would swiftly form a new “America Party.”

“If this insane spending bill passes, the America Party will be formed the next day,” he wrote in one of several Monday posts to his 220 million followers on X. “Our country needs an alternative to the Democrat-Republican uniparty so that the people actually have a VOICE.”

By the evening, Mr. Musk was committing to specific action, saying that he would support Representative Thomas Massie of Kentucky, among the most prominent holdouts against Mr. Trump’s bill. Though various G.O.P. factions have voiced concerns about the legislative package, potentially imperiling its passage, almost every Republican member in Congress supports some version of it.

At one point in the evening, Mr. Musk wrote that nearly the entire House and Senate G.O.P. “will lose their primary next year if it is the last thing I do on this Earth” — a tall task for even the world’s richest person, who donated nearly $300 million to Republican candidates in the 2024 election.

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