Elon Musk ramped up his criticism of the massive tax and domestic policy package working its way through the Senate on Monday, suggesting he could support primary challenges against Republicans who vote for the Trump-backed bill.

"Every member of Congress who campaigned on reducing government spending and then immediately voted for the biggest debt increase in history should hang their head in shame!" the billionaire and onetime Trump ally wrote in a post on X. "And they will lose their primary next year if it is the last thing I do on this Earth."

In a separate post, Musk said he will support Rep. Thomas Massie, a Kentucky Republican who voted against the bill when it passed the House, leading Mr. Trump to back a primary challenge.

Musk also suggested starting "a new political party that actually cares about the people" on Monday, an idea he first floated earlier this month. He said if the bill passes, a new political party — which he calls the "America Party" — "will be formed the next day."

Musk's latest swipes came as the Senate worked to advance the One Big Beautiful Bill Act, holding a marathon series of amendment votes on Monday. The Senate still needs to vote on final passage, and the House will need to approve the Senate's changes, before a self-imposed July 4 deadline to send the bill to Mr. Trump's desk.

It's unclear how serious Musk is about the apparent threat to back primary challenges against supporters of the legislation — or what support, if any, he could offer to challengers. But it suggests Musk could remain involved in politics, after spending upwards of $250 million to help elect Mr. Trump and other Republicans last year. The Tesla CEO indicated last month he planned to dial back his political spending for now, saying, "I think I've done enough."

The vast majority of House and Senate Republicans have backed the bill, though some lawmakers have withheld their support, objecting to its Medicaid cuts or arguing it doesn't go far enough to slash spending. Mr. Trump has applied intense pressure to Republicans and lashed out at the remaining holdouts: In a statement last weekend, the White House said "failure to pass this bill would be the ultimate betrayal." 

Musk has railed against the legislation on and off for weeks. The billionaire blasted the bill in early June, calling it a "disgusting abomination" — igniting a dayslong feud between President Trump and the world's richest man that seemed to signal the end of a once close partnership.

The billionaire, who once led the Trump administration's Department of Government Efficiency, or DOGE, backed down in mid-June and acknowledged some of his attacks on Mr. Trump "went too far." He remained fairly quiet about the legislation for weeks, but his criticism has intensified in recent days, as lawmakers rush to finalize the bill by the end of the week.

Mr. Trump lashed out at Musk in an early Tuesday morning Truth Social post that noted Tesla has benefitted from federal electric vehicle tax credits — a program the One Big Beautiful Bill Act would phase out. The president also suggested DOGE should "take a good, hard, look at" other subsidies Musk has benefitted from. 

"Elon may get more subsidy than any human being in history, by far, and without subsidies, Elon would probably have to close up shop and head back home to South Africa. No more Rocket launches, Satellites, or Electric Car Production, and our Country would save a FORTUNE," Mr. Trump wrote.

Musk responded to Mr. Trump's message: "I am literally saying CUT IT ALL. Now."

Why is Musk opposed to Trump's "big, beautiful bill"?

Many of Musk's criticisms zero in on the bill's price tag, calling its spending levels "insane" and attacking a provision in the Senate version of the bill that hikes the debt ceiling by $5 trillion. 

But the billionaire has also panned the bill's cuts to green energy tax credits and excise tax on certain renewable energy projects, calling those provisions "utter madness" that could "destroy millions of jobs in America and cause immense strategic harm to our country."

Those provisions could directly impact Musk's electric carmaker, Tesla. The phaseout of electric vehicle tax credits may cost the company $1.2 billion, an analysis by JPMorgan Chase estimated. Tesla's solar power and energy storage businesses also benefit from government incentives, and ending those programs "may harm our business" by "making our products less competitive for customers," the company disclosed in an annual report filed earlier this year.