A federal judge in Washington expressed astonishment Tuesday at arguments raised by President Donald Trump's lawyers seeking to block his accounting firm from turning over years of financial records to the Democratic-controlled House Oversight and Reform Committee and seemed to signal a swift ruling in favour of lawmakers.

US District Judge Amit Mehta fired pointed questions at the president's lawyers, who argued in a 22 April lawsuit that the committee's sweeping subpoena to Mazars USA for the financial records of Mr Trump and various associated entities since 2011 was not "a valid exercise of legislative power."

Lead Mr Trump attorney William Consovoy accused Democrats of "assuming the powers of the Department of Justice" on a partisan crusade, arguing that "this is about the House being dissatisfied with the president, and wanting to prove by any means possible that he has done things wrong."

Douglas Letter, general counsel of the House of Representatives, replied that Mr Trump's claim of freedom from congressional oversight marked "a total, basic and fundamental misunderstanding" of the Constitution, saying he would pronounce Congress "a nuisance . . . getting in his way while he's trying to run the country."

The hearing Tuesday was the first courtroom confrontation in a wave of legal battles waged by the president to shield his personal finances from investigators, including congressional Democrats, state lawmakers and regulators. Several fights are asking judges to weigh in on the constitutional separation of power between coequal executive and legislative branches.

Judge Mehta asked: "Say a president was involved in some corrupt enterprise, you mean to tell me because he is the president of the United States, Congress would not have power to investigate?" (File photo) (DON EMMERT/AFP/Getty Images)

Mr Mehta, a 2014 appointee of President Barack Obama, challenged some of Mr Trump's assertions in questions to his legal team.

"Is it your position that whether the president has properly reported his finances (under federal disclosure laws), that's not subject to investigation by Congress?" Mr Mehta asked.

"Say a president was involved in some corrupt enterprise, you mean to tell me because he is the president of the United States, Congress would not have power to investigate?" Mr Mehta asked, continuing, what if "we're talking about a presidential violation of a constitutional prohibition that only Congress has authority to approve?" such as the acceptance for emoluments or gifts from a foreign government.

Mr Consovoy answered yes, saying determining whether a president properly disclosed his finances was a "pure law enforcement function," not a matter for Congress, whose fundamental duty he said is in writing bills.

And if Congress did seek to enact legislation, whether to tighten ethics requirements, fight public corruption or enforce a constitutional prohibition on acceptance of foreign emoluments, Mr Consovoy said, "I don't think there is any permutation of any of these proposals that could pass constitutional muster," because they would interfere with the president's execution of his duties, or add qualifications for his office.

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"There's not a single Supreme Court or appellate case since 1880 that has found Congress overstepped its legislative authority by issuing a subpoena," Mr Mehta observed at another point.

The lawsuit in Washington was brought by Mr Trump and several of his businesses against House Oversight Committee Chairman Elijah Cummings and Mr Trump's accounting firm Mazars USA.

Congressional Democrats accuse Mr Trump of trying to stonewall lawmakers' oversight until after the 2020 election, while the president's team counters that it will not tolerate a campaign of "congressional presidential harassment."

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Mr Mehta gave both sides until Saturday to file any additional submissions and promised a ruling "promptly, consistent with the gravity of the issues" involving the balancing of powers between Congress and a president.

Mr Mehta did not say when he would rule, but he noted that a different House panel, the Financial Services Committee, is set to appear in federal court in Manhattan on May 22 to defend other subpoenas for Mr Trump's bank records issued to Deutsche Bank AG and Capital One Financial. Mr Trump and his three eldest children and companies have sued to quash the subpoenas.

Mr Mehta said at the end of Tuesday's 80-minute hearing that he would also make clear in his opinion whether he would stay a subpoena, if he upholds it, to give the president time to appeal.

Mr Mehta also questioned aspects of the House's legal arguments, asking whether Congress was claiming blanket authority to investigate matters by claiming a duty to "inform the public."

"It really does open the door to the accusation, perhaps valid or not, that this really is an effort - if not to harass the president - then to get into his private affairs for political purposes, if there is no clear line as to what this investigation is about," Mr Mehta said. "How do I draw lines to test, even if it's a soft test, the validity of what you're doing? "

Letter said Congress' function to "inform the public" about the president is a valid basis for the subpoena, citing past investigations into the 2001 terrorist attacks on New York and Washington, the origins of the Iraq War and former President Bill Clinton's Whitewater land dealings.

But he said the House also had obvious legislative purposes to oversee the function of laws governing the financial disclosure of public officials and avoidance of conflicts of interests, the handling of presidential records and the prevention of foreign governments holding hidden financial influence over American elected officials.

"We need to know that," Letter argued. "We need to know, is the president of the United States beholden to foreign interests, because they can hold things over his head?"

Pressed by Mr Mehta to come up with some hypothetical demand by Congress that would go too far, Letter speculated. "I want to see the president's diary when he was 7 years old, or when she was 12 years old. That would probably stretch my argument to the breaking point."

Cummings' panel last month subpoenaed Mazars, seeking documents to corroborate testimony by the president's former lawyer Michael Cohen, who said at a congressional hearing that Mr Trump intentionally misreported the value of his assets for personal gain.

Other House panels have requested Mr Trump's banking records and tax returns, while his company also faces enquiries from New York state regulators and is defending itself in two lawsuits alleging that the company violates the Constitution by doing business with foreign governments.

Mazars attorney Henry Schuelke has said the firm took no position on the case.