Nancy Pelosi, the Democrat speaker of the House, claimed Donald Trump's tax returns raised national security questions.
Mrs Pelosi, said the American public deserved to know to whom he owed money.
The New York Times reported on Sunday night that it has seen two decades of the president's tax returns.
Mr Trump called the report "totally fake news".
According to the newspaper Mr Trump had $421 million of debt coming due in the next several years.
Mrs Pelosi said it was not clear who held the debt and whether it involved different countries, which could hold leverage over the Republican president.
She told MSNBC: "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.
“We take an oath to protect and defend. 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.
"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."
Speaker Pelosi: New report that President Trump has more than $300M in loans coming due in the next few years exhibits a “national security issue" that raises questions about whether foreign nations or individuals could have leverage over the president. https://t.co/5Fqn1ob3i6— MSNBC (@MSNBC) September 28, 2020
Mr Trump said he had many more assets than debt.
He said: "I am extremely under leveraged - I have very little debt compared to the value of assets."
Mr Trump departed from decades of tradition among presidents and presidential candidates when he refused to release his federal income tax returns the first time he ran for president in 2016.
He has said it is because he is under audit by the Internal Revenue Service but the agency has said there is no reason he cannot release his taxes while under audit.
The Republican president will face Democratic rival Joe Biden on Tuesday night for their first presidential debate.