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House Democrats should make it their top priority to obtain Donald Trump's tax returns and release them to the public, a progressive tax group argues in a new report obtained by NBC News.

Americans for Tax Fairness, a left-leaning organization backed by organized labor, says in a 21-page white paper that securing Trump's tax returns "is an important public policy objective and a legitimate exercise of Congress's oversight authority."

As NBC News reported last month, Democrats plan to use a 1924 law to demand that Treasury Secretary Steve Mnuchin turn over Trump's tax records to Richard Neal, the chairman of the House Ways and Means Committee.

The law appears to allow the committee to vote to make the returns public. Democrats say they expect that Mnuchin will not hand over the returns without a court fight, however.

President Donald Trump during a meeting with members of his Cabinet at the White House on Jan. 2, 2019. Michael Reynolds / EPA

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Trump was the only major party nominee in four decades other than Gerald Ford not to release his tax returns—and Ford released summaries of his returns. Trump has said he won't release them because he is under audit.

House Democrats are pushing legislation that would require presidential candidates to release a decade of tax returns, aides tell NBC News, but that bill's chances of becoming law are uncertain.

In a report designed to make the case for forcing the release of Trump's taxes, Americans for Tax Fairness argues that the president's returns would be valuable as a guide "to the complex financial structures and tax loopholes used by the wealthy," and also "as the best source of answers to vital questions about Donald Trump's presidency, including whether there are any conflicts of interest and/or foreign influences that could be adversely affecting his exercise of official duties."

The returns could also shed light on whether Trump or his family benefited from the tax cuts he signed into law in 2017, the report says, and on a New York Times investigation alleging that Trump committed tax fraud.

The returns are also likely to provide information relevant to whether Trump's public actions enhanced his private wealth, particularly in relation to his luxury hotel a few blocks from the White House, the report says.

"The president has shown inexplicable deference to the leaders of foreign powers in conflict with the United States, including Russia and Saudi Arabia, raising the possibility that his business connections in those countries are influencing his conduct of American foreign policy and threatening national security," the report says. "His tax returns and those of his business entities could provide evidence of foreign business connections that jeopardize his independence."

"Obtaining the president's tax returns is not some fishing expedition," said Frank Clemente, executive director of Americans for Tax Fairness. "It's a necessary exercise in the oversight responsibility of Congress. The public has a right to know if the policies the president advocates are what he believes are in the best interest of the country or in the best interest of his personal finances."

The White House did not immediately respond to a request for comment by NBC News.

Legal experts tell NBC News that special counsel Robert Mueller almost certainly has obtained President Trump's tax returns and other private business information as part of his investigation into whether Trump or his campaign coordinated with the 2016 Russian election interference operation.

But Mueller would not be able to make that information public unless it was relevant to a criminal case.