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A leading Republican critic of Donald Trump’s flagship tax bill in the US Senate has warned of growing concerns within the party about its fiscal impact, ramping pressure on the president to back deeper spending cuts.

In a wide-ranging interview with the Financial Times on Tuesday, Republican Senator Ron Johnson said he would condition his vote on ironclad assurances that the White House slash more federal spending with a second major bill before next year’s midterm elections.

“I need to be assured that there will be another bite of the apple. We will have another must-pass budget reconciliation process here in Congress,” said the Wisconsin senator. “This is our opportunity, and I just don’t think we can blow it . . . I am digging my heels in.”

Johnson is among the loudest voices on Capitol Hill to have raised concerns about the cost of what Trump has called his “big, beautiful bill”, and his vote could kill its progress in Congress.

He has called for government spending to be cut back to pre-pandemic levels — and told the FT that opposition to the legislation was growing in the Senate.

“There are plenty of senators that are concerned about this,” he said. “We are supportive of the direction of the bill. It just doesn’t go far enough” and “more and more people are aware of these kinds of problems”.

Johnson said his “case” had been “bolstered” by Elon Musk, Trump’s billionaire backer who spectacularly fell out with the president last week after he derided the tax bill as a “disgusting abomination”.

The bill would extend Trump’s 2017 tax cuts while cutting social programmes and raising the debt ceiling, or limit on government borrowing, by $5tn. The non-partisan Congressional Budget Office said last week it would add $2.4tn to US debt by 2034.

Johnson, who spent decades working for a family-owned plastics manufacturer before being elected to the Senate in 2010, wields outsized influence on Capitol Hill at a time when Republicans control the Senate by a 53-47 margin.

Several other Republican senators — including fellow fiscal hawks Rand Paul of Kentucky, Mike Lee of Utah and Florida’s Rick Scott — have also raised alarm about the sweeping legislation.

Johnson said investor reaction to the legislation was also worrying.

“Look at the bond market. These are pretty smart, sophisticated people, and they are a little more reluctant to buy US bonds, or are ploughing money into gold or other currencies,” Johnson said. “That ought to concern people.”

Asked whether Federal Reserve chair Jay Powell should lower interest rates as Trump has demanded, Johnson said it was no longer up to the US central bank boss.

“It is beyond his control. The bond buyers are controlling this now.”

The spending bill narrowly passed through the House of Representatives last month and Trump wants to enact it by July 4. Johnson said the timeline was “pretty optimistic”.

The senator spoke to the FT in his Capitol Hill office hours after what he called a “very good meeting” with vice-president JD Vance and National Economic Council director Kevin Hassett.

The White House was taking his concerns about the US deficit “seriously”, he said.

“[Trump] knows I know my numbers. He knows I’m sincere about this, and that is why he has directed his economic team to work with me,” he said.

Johnson, 70, described himself as a “big supporter” of Trump, calling him a “unique figure”, but added: “My higher loyalty is to my kids and grandkids, and we are literally mortgaging their future. That’s immoral.”

Johnson also dismissed the threat of a Trump loyalist running against him in a Republican primary, saying he had not decided whether to run for re-election when his current six-year term is up in 2028.

“I think that’s probably the miscalculation from the White House . . . it is a lot easier to twist the arms of House members [than senators],” he said. “A primary from President Trump . . . in today’s Republican party, that is probably a death sentence to your . . . political career.”

But he added: “I would be happy to go home. I would probably prefer to go home.”