United States House Speaker Nancy Pelosi says poses a "national security question".
Mr Trump owes hundreds of millions in debts amid chronic business losses that he uses to avoid paying taxes, the New York Times reported on Sunday, citing tax data it had obtained going back two decades.
It also showed Mr Trump paid only $750 USD ($1,059 AUD) in federal income tax in both 2016 and 2017, and in several other years, nothing.
Ms Pelosi said on Monday she was concerned by the revelations contained in the Times report.
“This president appears to have over $400 million in debt. To whom? Different countries? What is the leverage they have? So for me, this is a national security question,” she told MSNBC.
“This president is commander in chief. He has exposure to the tune of hundreds of millions of dollars, to whom?
“The public has a right to know.”
Mr Trump on Sunday, without proof, rubbished the claims and told reporters at the White House it was “fake news”.
He also denouncing the Times report, and boasted he had “extraordinary assets” and a “very IMPRESSIVE” financial statement.
"I paid many millions of dollars in taxes but was entitled, like everyone else, to depreciation & tax credits," he tweeted.
"I am extremely under leveraged - I have very little debt compared to the value of assets."
The reports about Mr Trump’s tax returns has also seen an old tweet of his resurface in which he criticises his predecessor Barack Obama for his 20.5 per cent tax rate.
“@BarackObama who wants to raise all our taxes, only pays 20.5% on $790k salary. Do as I say not as I do,” Mr Trump wrote in 2012, which is one of the years the Times report says he paid no tax.
Another Democrat, New York Congresswoman Alexandria Ocasio-Cortez, slammed Mr Trump in the wake of the Times report for reportedly paying less tax “than waitresses and undocumented immigrants".
“In 2016 & ‘17, I paid thousands of dollars a year in taxes *as a bartender.* Trump paid $750. He contributed less to funding our communities than waitresses & undocumented immigrants,” she wrote on Twitter Sunday.
“Donald Trump has never cared for our country more than he cares for himself. A walking scam.”
Mr Trump is the first president in years not to make public his tax returns, claiming he can't because he is under audit.
On Tuesday, he will face Democratic nominee Joe Biden in their first presidential debate.
Additional reporting by AFP.
Watch the first of three presidential debates LIVE on SBS and SBS News social platforms from 11am tomorrow.
