Washington DC - The White House on Wednesday blasted a prediction that Donald Trump's policy mega-bill could send the deficit soaring, as Elon Musk doubled down on his criticism of the president's plans.

The non-partisan Congressional Budget Office said Trump's "one big, beautiful bill" – the centerpiece of his domestic agenda – would add a giant $2.4 trillion to the US deficit by 2034.

Trump's combative Deputy Chief of Staff Stephen Miller slammed the independent watchdog on social media as "lefty" – echoing the administration's frequent line of attack against its opponents.

"We are in a very good place with the bill," said Russ Vought, head of the White House's Office of Management and Budget, which did the sums for Trump's bill. He told reporters on a call that the congressional office's prediction "does not reflect reality."

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But the package, which could define Trump's second term and make or break Republican prospects in the 2026 midterms, is getting a rough ride in Congress.

The plan would fund an extension of Trump's 2017 tax relief for the wealthy from his first term in the White House by piling on debt and cutting social welfare for the poorest Americans.

Tech billionaire Elon Musk, who departed as head of Trump's cost-cutting Department of Government Efficiency last week amid growing tensions, issued fresh criticism on Wednesday.

"KILL the BILL," the South African-born tycoon posted on his X social network, following it up with a meme showing the blood-soaked Quentin Tarantino movie Kill Bill and its star Uma Thurman.

Musk had called the bill a "disgusting abomination" on Tuesday.

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The bill passed the House of Representatives last week but now faces a difficult path through the US Senate.

A group of senators visited the White House on Wednesday as the negotiations entered a crucial stage, with Trump urging the upper chamber to vote on it by July 4.

"Failure is not an option," Republican Senate Majority Leader John Thune told reporters outside the West Wing.

Asked about Musk's claims that it would balloon the deficit, he replied: "We believe the opposite."