With Democrats now controlling the House and holding the legal key to seeking President Donald Trump's tax returns, Republican lawmakers are invoking privacy in defending Trump's flank.

At an oversight hearing on Thursday, lawmakers examined proposals to compel presidents and presidential candidates to make years of their tax returns public.

And they discussed the authority under current law for the head of the House Ways and Means Committee - now Democratic Representative Richard Neal - to make a written request for any tax returns to the Treasury secretary.

The law says the Treasury chief "shall furnish" the requested information to members of the committee for them to examine behind closed doors.

Republicans accused the Democrats of using powers in the tax law to mount a political witch hunt for Trump's tax returns.

"In reality, this is all about weaponising our tax laws to attack a political foe," Representative Jackie Walorski of Indiana said at the hearing by the Ways and Means oversight subcommittee.

Priority

Getting Trump's returns has been high on the Democrats' list of priorities since they won control of the House in November's midterm elections, but asking for them will probably set off a huge legal battle with his administration.

The Democrats tried and failed several times to obtain Trump's returns as the minority party in Congress, seeking to shed light on his complex financial dealings and potential conflicts of interest.

Their newly energised leftward wing is pushing Neal to set the quest in motion, and fast.

Thursday's hearing appeared to set the table for the move by examining the legal foundations.

"A strong case is being built," William Tranghese, an aide to Neal, told The Associated Press this week. He said Neal is consulting with lawyers for the House "to determine the appropriate legal steps to go forward with this unprecedented request".

Representative John Lewis, (a Democrat from Georgia) chair of the oversight subcommittee, said the American public is intensely interested in the subject.

"We ask the question: Does the public have a need to know that a person seeking or holding the highest office in our country obeys the tax laws?"

George Yin, a professor of law and taxation at University of Virginia Law School, testified to the panel that he doesn't see any "wiggle room" in the law for the Treasury secretary to refuse Neal's request for Trump's returns.

If the Trump administration refused the request, "We would be in uncharted territory," Yin said.

Legal battle

The legal battle that could ensue over Trump's tax filings would be unprecedented. It could take years to resolve, possibly stretching beyond the 2020 presidential election.

Representative Mike Kelly of Pennsylvania, the subcommittee's senior Republican, accused the Democrats of gearing up to obtain the president's returns - and release them.

"Congress is prohibited by law from examining and making public the private tax returns of Americans for political purposes," said Kelly.

"Such an abuse of power would open a Pandora's box. It would set a very dangerous precedent."

The tax returns of House Speaker Nancy Pelosi, other lawmakers or federal employees could be in jeopardy, he warned.

But Bill Pascrell (a Democrat from New Jersey) insisted that no one, including the president, is above the law. "The law is on our side," he said.

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