Next year’s midterm elections could end up being a proxy war between President Trump and Elon Musk now that the world’s richest man is saying he’ll back primary challengers against all GOP lawmakers who support the “big beautiful bill.” Mr. Musk kicked off his potential parade of endorsements on Monday, saying he will back Congressman Thomas Massie. 

Mr. Musk went on a tear on Monday afternoon, saying that the One Big Beautiful Bill Act is a costly mistake that will add trillions of dollars to the deficit. He said it is the product of America’s “PORKY PIG PARTY.”

He has raised concerns about the $5 trillion debt limit increase that is included in the Republican-backed bill. The Senate has been working on the legislation for the last four weeks, and is expected to pass it some time on Monday. 

“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!” Mr. Musk wrote Monday. “And they will lose their primary next year if it is the last thing I do on this Earth.”

“They just pretend to be two parties. It’s just one uniparty in reality,” Mr. Musk wrote later in the day, including a graph showing the national debt over time. 

The one Republican member of Congress he has promised to back next year is Mr. Massie, who voted against the bill last month and has refused to even consider voting for the version the Senate is now crafting, which will be sent back to the House in the coming days. 

After Mr. Musk threatened to back primary challengers to GOP incumbents, a former libertarian lawmaker, Congressman Justin Amash, wrote in the comments section to ask the world’s richest man to support Mr. Massie in his re-election effort. 

“Please support @RepThomasMassie,” Mr. Amash wrote under Mr. Musk’s original post. “The establishment is working to primary him because he’s a genuine fiscal conservative and opposes the Big, Bloated Scam.”

“I will,” Mr. Musk then replied. 

Mr. Trump issued a special threat to Mr. Musk early Tuesday morning, saying that he may have DOGE “take a look” at his former advisor’s companies. 

“Elon Musk knew, long before he so strongly Endorsed me for President, that I was strongly against the EV Mandate,” Mr. Trump wrote on Truth Social, referring to the electric vehicle tax credit. “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.” 

“Perhaps we should have DOGE take a good, hard, look at this? BIG MONEY TO BE SAVED!!!” Mr. Trump wrote. 

The only other Republicans who are now publicly opposed to the “big beautiful bill” are Senators Tillis and Paul, though Mr. Tillis is retiring next year and Mr. Paul isn’t up for re-election until 2028. Congressman Warren Davidson voted against the original House bill, though he is open to supporting the Senate version depending on what is in the legislation. Mr. Davidson also has not yet drawn the president’s ire the way Mr. Massie has. 

Mr. Musk also on Monday criticized members of the House Freedom Caucus, who are critical of the Senate bill as it takes shape though have not yet taken a firm stance against it. 

“How can you call yourself the Freedom Caucus if you vote for a DEBT SLAVERY bill with the biggest debt ceiling increase in history?” Mr. Musk asked rhetorically on Monday. In his post, he tagged the chairman of the conservative group, Congressman Andy Harris, and an outspoken member of the group, Congressman Chip Roy. 

Mr. Roy voted for the House bill in May while hoping that the Senate could deliver a more fiscally sound version. He has yet to see that result materialize, however. 

In an interview on Monday with a conservative commentator, Dana Loesch, Mr. Roy said the current text of the Senate bill “misses the mark” on winning conservative results. 

“My read of the bill — as I understand it currently coming out of the Senate — is that it will miss the mark on the House framework by about $650 billion over 10 years, plus interest. It means we will add well over a trillion dollars to the deficit. I didn’t come to Washington to do that,” Mr. Roy said. “I’m not ready to vote for this bill as it currently is.”