The tech billionaire followed his criticism with a threat aimed at Republicans.

“In November next year, we fire all politicians who betrayed the American people,” he wrote in another X post.

It's a sharp shift for Mr. Musk, the world's richest person, who spent at least $250 million supporting Mr. Trump's campaign last year. He previously pledged to help defeat Republican lawmakers deemed insufficiently loyal to Mr. Trump, but now he's suggesting voting them out if they advance the president's legislative priority.

However, it's unclear how Mr. Musk will follow through on his criticism. He recently said he would spend “a lot less” on political campaigns, though he left the door open to political involvement "if I see a reason."

The tech titan's missives could cause headaches for Republicans on Capitol Hill, who face conflicting demands from Mr. Trump and their party's wealthiest benefactor.

Alex Conant, a Republican strategist, said, “It’s not helpful” to have Mr. Musk criticising the legislation, but he doesn’t expect lawmakers to side with Musk over Trump.