In the latest stand-off between the Trump administration and Democrats on Capitol Hill seeking to exert leverage, Steven Mnuchin revealed he was refusing the request as it lacked “a legitimate legislative purpose”.

Democrats have long sought to obtain a copy of the president’s tax returns, in the belief it would provide potentially damaging information about the way he has operated his finances and his businesses both before and after entering the White House.

There is no law saying presidents must do so, though most in recent history have made them public. While campaigning for the White House, Mr Trump said he would release them once the Internal Revenue Service (IRS) had completed an audit he said was being carried out.

Commentators pointed out that tax returns could be made public even during an audit, and some have suggested the president simply enjoyed antagonising his opponents by declining to do so.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

The latest request for details of the president’s returns had been made last month by congressman Richard Neal, the Democratic chairman of the way and means committee of the House of Representatives. He did so on the basis of a law that says the IRS “shall furnish” the returns of any taxpayer to a handful of senior legislators, among them the chair of that committee.

But in a letter made public on Tuesday, Mr Mnuchin made clear it did not intend to do so.

“Out of respect for the deadlines previously set by the committee, and consistent with our commitment to a prompt response, I am informing you now that the department may not lawfully fulfil the committee’s request,” he wrote.

He said the treasury department would work with Mr Neal “to provide information concerning the committee’s stated interest in how the IRS conducts mandatory examinations of presidents, as provided by the internal revenue manual”. “If the committee is interested, we remain committed to providing such an accommodation,” Mr Mnuchin said.

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Mr Neal, who represents Massachusetts’ first congressional district, said in a statement: “I will consult with counsel and determine the appropriate response.”

The expectation now is that the issue will be further spun out by means of a legal tussle. Democrats could seek to subpoena the IRS for the president’s tax returns or to file a lawsuit.

Mr Neal has said his committee was looking into the effectiveness of IRS mandatory audits of tax returns of all sitting presidents, a way to justify his claim that the panel had a potential legislative purpose.

Democrats are confident in their legal justification and say the president is stalling in an attempt to punt the issue past the 2020 election.

Mr Trump has said he was elected by the American public without disclosing his returns. Last month, he doubled down on this claim that returns cannot be released while an audit is under way, something experts say is not true.

“There is no law, as you know, I got elected last time with the same issue and while I’m under audit, I won’t do it,” he told reporters. “If I’m not under audit, I would do it, I have no problem with it, but while I’m under audit, I would not give my taxes.

“I have no obligation to do that while I’m under audit. And no lawyer would tell you to release your tax returns while you’re under audit.”

Mr Trump is first major candidate since 1976 to not make any of his full returns public. President Gerald Ford released a tax summary that year, but not his full records.