For 130 days, tech billionaire Elon Musk was a "special government employee" working inside the Trump administration to slash what he said were wasteful government programmes and bloated departments.

Now, after his exit last week, he is back on the outside. And it is the Trump administration – and the massive spending legislation the president is currently pushing – that is drawing the ire of this new, seemingly unshackled Musk.

In a series of social media posts on Tuesday, he took aim at Trump's signature tax and spending bill that the Senate is considering after House Republicans passed it last month. After saying he couldn't "stand it anymore", Musk called the bill a "disgusting abomination".

"Shame on those who voted for it," he said. "You know you did wrong."

And on Wednesday afternoon, he went a step further, calling on voters to contact their representatives in Congress to voice their opposition. "Bankrupting America is not OK!" he wrote on X. "KILL the BILL."

Musk's diatribe against the Trump-backed bill, which includes huge tax breaks and more investment in defence, comes after he spent months pushing for cuts to government spending.

In his posts, he said the legislation would add to the US budget deficit and saddle Americans with debt. He also warned Republicans who supported, or are planning to support, the bill that "we [will] fire all the politicians who betrayed the American people" in next year's midterm elections.

And it is that line that could be the most concerning for Republicans.

In the House, all but three Republican members backed the bill, which passed by a single vote over unified Democratic opposition.

If Musk turns on the majority of Republicans - after spending hundreds of millions to support their campaigns last year - it could cause headaches for incumbents worried about facing challenges in party primaries and represent a devastating blow to Republican hopes of retaining control of Congress for the second half of Trump's second term.

Although Musk had offered more measured criticism in an interview last week, this week marks a stark change from the chummy meeting he had with Trump in the Oval Office on Thursday, where he stood, smiling, as Trump expounded on the importance of what he said will be his defining legislative achievement.

The president has yet to respond to Musk, but he is sure to be asked about the comments. So far, however, the White House has been handling the situation delicately.

According to US media reports, the White House is attempting to explain Musk's vocal opposition as a response to the bill's cuts to green energy subsidies, including tax credits for electric vehicles like Musk's Tesla cars.

Axios, meanwhile, reported that another source of tension is Trump's decision to pull the nomination of Jared Isaacman – a Musk ally – to be head of the US space agency Nasa.

In the halls of Congress on Wednesday, established Trump allies were treading a similarly careful path.

"The bill can be made better, but it's not an abomination," Senator Lindsey Graham, a leading figure within the Republican Party and loyal backer of Trump, told the BBC.

"Let's pass this bill," he said. "We can make additional cuts to spending as necessary, but I want to put some points on the board. Get the tax cuts, make them permanent, get money, do the border. But [Musk] is right, we should cut more."

Speaker Mike Johnson, meanwhile, reportedly spent time pushing back on Musk's criticism of the bill in a closed-door meeting with Republicans. He also tried to reassure members over the billionaire's threats to oust supporters of the bill, according to Politico.

On Tuesday, White House Press Secretary Karoline Leavitt said that Trump "already knows" Musk's view on the legislation. "This is one big, beautiful bill, and he's sticking to it," she added.