The top-ranked Democrat in Washington has called President Donald Trump's alleged tax avoidance a question of "national security".
Nancy Pelosi, Speaker of the House of Representatives, questioned whether Mr Trump owed money to foreign interests, following a disclosure of his financial records by the New York Times.
Among the revelations is that Mr Trump paid $750 (£580) in federal income tax in 2016 and 2017.
Mr Trump called the report "fake news".
Speaking on NBC, Ms Pelosi said the report showed that "this president appears to have over $400m in debt".
"To whom? Different countries? What is the leverage they have?" she asked, adding: "So for me, this is a national security question."
"The fact that you could have a sitting president who owes hundreds of millions of dollars that he's personally guaranteed to lenders, and we don't know who these lenders are," she said, and suggested that Mr Trump may be indebted to Russian President Vladimir Putin. "What does Putin have on the president politically? Personally? Financially?"
According to the explosive report in the New York Times – which says it obtained tax records for Mr Trump and his companies over two decades – Mr Trump paid no income taxes at all in 10 of the previous 15 years. It adds that the president is personally responsible for more than $300m in loans, which will come due in the next four years.
It does not suggest Mr Trump received any previously unknown income from Russia, though it revealed that the president has earned some money from foreign sources.
The records reveal "chronic losses and years of tax avoidance", it says.
"Actually I paid tax. And you'll see that as soon as my tax returns – it's under audit, they've been under audit for a long time," Mr Trump told reporters on Sunday after the story was published.
"The IRS [Internal Revenue Service] does not treat me well… they treat me very badly," he said.
Mr Trump has faced legal challenges for refusing to share documents concerning his fortune and business. He is the first president since the 1970s not to make his tax returns public, though this is not required by law.
The New York Times said the information scrutinised in its report was "provided by sources with legal access to it".
The report came just days before Mr Trump's first presidential debate with his Democratic rival, Joe Biden, and weeks before the 3 November election.
What are the key claims?
The Times said it reviewed tax returns relating to President Trump and companies owned by the Trump Organization going back to the 1990s, as well as his personal returns for 2016 and 2017.
It said the president paid just $750 in income taxes in both 2016 and 2017, while he paid no income taxes at all in 10 of the previous 15 years, "largely because he reported losing much more money than he made".
Before becoming president, Mr Trump was known as a celebrity businessman and property mogul, building an image of a hugely successful self-made billionaire which could be dented by the latest revelations, observers say.
But the newspaper says his reports to the IRS "portray a businessman who takes in hundreds of millions of dollars a year yet racks up chronic losses that he aggressively employs to avoid paying taxes".
According to the US Bureau on Labor Statistics, the average American household paid $9,302 in federal income tax in 2018, on an average earnings figure of $78,635.
In an annual financial disclosure that he is required to make as president, President Trump said he made at least $434.9m in 2018. The newspaper disputes this, alleging that his tax returns show the president had instead gone into the red, with $47.4m in losses.
The Trump Organization joined the president in denying the allegations in the report. The company's chief legal officer, Alan Garten, told the Times that "most, if not all, of the facts appear to be inaccurate".
"Over the past decade, President Trump has paid tens of millions of dollars in personal taxes to the federal government, including paying millions in personal taxes since announcing his candidacy in 2015," he said.
By using the term "personal taxes", the New York Times points out, Mr Garten appears to be conflating other federal taxes paid by Mr Trump – such as social security, Medicare and taxes for people who work in his household – with federal income tax.
What else does the report say?
The newspaper also claims that "most" of Mr Trump's biggest businesses – such as his golf courses and hotels – "report losing millions, if not tens of millions, of dollars year after year".
"That equation is a key element of the alchemy of Mr Trump's finances: using the proceeds of his celebrity to purchase and prop up risky businesses, then wielding their losses to avoid taxes," it says.
The newspaper also alleges that some of President Trump's businesses have received money from "lobbyists, foreign officials and others seeking face time, access or favour" from the president.
The New York Times says it used tax records to find out how much income the president makes from his companies overseas, alleging that he made $73m in revenue from abroad in his first two years at the White House.
Much of that came from his golf courses in Ireland and Scotland, but the newspaper says the Trump Organization also received money "from licensing deals in countries with authoritarian-leaning leaders or thorny geopolitics".
Licensing deals allegedly Mr Trump netted $3m from the Philippines, $2.3m from India and $1m from Turkey.
Mr Trump is said to have made $427.4m in total up until 2018 in revenues from The Apprentice US series, as well as from branding deals where organisations paid to use his name. He also made $176.5m in profit by investing in two office buildings, it is alleged.
The president allegedly paid almost no taxes on these sizeable earnings, because he reported that the businesses that he owns and runs made significant losses.
The Times also alleges that Mr Trump reduced his taxable income by paying consulting fees to Ivanka Trump, his daughter and senior adviser. In financial disclosures filed in 2017, she reported receiving $747,622 from a consulting company she co-owned. Consulting fees of exactly the same amount were claimed as tax deductions by the Trump Organization for hotel projects, the Times says.
Mr Trump is also said to have written off more than $70,000 in hair-styling costs as business expenses during his time on The Apprentice.
The report claims President Trump has been making use of a tax code that enables business owners to "carry forward leftover losses to reduce taxes in future years".
For example, President Trump's largest golf resort, Trump National Doral, near Miami, has reportedly lost $162.3m since Read More – Source