The White House and Democrats in Congress are preparing for a lengthy legal fight as the US Treasury Department stares down a final deadline to release Donald Trump’s tax returns.

Democrats in the House Ways and Means Committee have requested six years of the president’s individual and business returns, and have set a final deadline for 5pm ET on Tuesday.

The committee informed Internal Revenue Service commissioner Charles Rettig in a letter that any failure to comply would be viewed as a denial.

The White House has vowed to fight the request, with acting chief of staff Mick Mulvaney saying that the returns would “never” be handed over to Congress. Treasury secretary Steven Mnuchin, meanwhile, has said that he intends to “follow the law”, and vowing to not allow the IRS to be “weaponised” in a political fight between the president and Congress.

But, while Mr Mulvaney has made clear he believes the tax returns will never see the light of day, Mr Trump and others in the White House have repeatedly attempted to explain the resistance by saying Mr Trump has been under audit.

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It is the same line that the president has been using since the campaign.

“As I understand it, the president’s pretty clear: Once he’s out of audit, he’ll think about doing it, but he’s not inclined to do so at this time,” Hogan Gidley, a White House spokesperson, said during an interview with Fox News.

Bill Pascrell, a leading Democrat fighting to secure the release of Mr Trump’s tax returns, said in a statement that the law is on their side, and that they will continue to fight to secure he financial documents that could shed light on potential conflicts of interest the president may have.

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“The law is on our side. The law is clearer than crystal. They have no choice: they must abide by (it),” Representative Bill Pascrell, who has been leading the Democratic push for Trump’s tax records, said in a statement to Reuters.

While running for president in 2016, Mr Trump broke from decades of tradition to become the first candidate for America’s highest office to not release detailed tax returns to the public.