Democrats will 'never' see Trump's tax returns, White House official vows

US President Donald Trump. Photo / AP

US President Donald Trump's acting chief of staff said that Democrats' efforts to see Trump's tax returns will "never" be successful, and "nor should they".

Mick Mulvaney, during an appearance on Fox News, characterised Democrats' efforts as politically motivated and claimed that voters had already decided in 2016 that they didn't care what is in Trump's returns.

"That's an issue that was already litigated during the election," Mulvaney said.

"Voters knew the President could have given his tax returns. They knew that he didn't, and they elected him anyway."

Trump broke precedent as a presidential candidate with his refusal to release his tax returns.

He said then, and continues to maintain, that he is unable to furnish them because he is under audit by the Internal Revenue Service.

Mulvaney seemed to contradict the President at one point in the show, saying that even under audit "you could always allow people to see it".

Democrats have cited Trump's decision on the tax returns as evidence that he has something to hide.

Last week, House Ways and Means Committee Chairman Richard Neal, sent a letter to the IRS formally requesting that six years of Trump's business and personal tax returns be turned over to Congress by Thursday.

A lawyer for Trump asked the Treasury Department to deny the Democrats' request, calling it a "gross abuse of power". Trump himself said he would take the fight over his tax returns all the way to the US Supreme Court.

Mick Mulvaney, the acting chief of staff at the White House. Photo / AP file

Mulvaney claimed that Democrats are overstepping. He said that although the law allows the IRS to hand over someone's returns under certain circumstances, a "political hit job is not one of those reasons".

The 4th-highest ranking House Democrat, Congressman Ben Ray Lujan of New Mexico, said they are exercising their oversight responsibilities.

"No other president in modern times has had to have their tax returns requested because ... they've all voluntarily shared them," Lujan said.