Donald Trump is positioning himself to become the first president in 40 years to refuse to release his tax returns in office.
Such a refusal would overturn a precedent maintained by every president since Jimmy Carter -- and extend Trump's refusal to make his returns public during the campaign.
White House senior adviser Kellyanne Conway said Sunday that Trump wouldn't do it.
"The White House response is that he's not going to release his tax returns," she told ABC's "This Week." "We litigated this all through the election. People didn't care."
In fact, a CNN poll in October found that 73% of registered voters surveyed thought Trump should release his tax returns for public review, including about half of Republicans.
In a tweet Monday, Conway said that Trump would not release the documents as long as he's under audit by the IRS. That was the same explanation he gave during the campaign, even though IRS audits do not restrict anyone from publishing tax returns.
"On taxes, answers (& repeated questions) are same from campaign: POTUS is under audit and will not release until that is completed," she said.
The audits won't end anytime soon. Presidents have been automatically audited for some time, said Joe Thorndike, a historian with Tax Analysts and director of the Tax History Project.
The IRS manual makes it clear: Individual income tax returns for the president and vice president "are subject to mandatory examinations."
There's no law that requires Trump to disclose the returns, since the papers are considered private information. But there's also no law that says he can't release them.
Norman Eisen, one of the lawyers bringing an ethics lawsuit against Trump, raised the possibility on CNN on Monday that a judge might order Trump's tax returns released as part of the case.
Why does release matter?
Even a portion of Trump's tax returns could show the American people some limited information about the billionaire's finances.
The top two pages of his 1040 and his Schedule A, for example, would reveal how much taxable income he made, how much he paid in taxes, his charitable contributions and whether he paid tax to any foreign governments.
The custom of releasing tax returns began with Richard Nixon, Thorndike said.
In 1973, while under audit and amid a controversy over his personal taxes, Nixon released returns dating back to when he took office in 1969. The scandal led to one of Nixon's most famous quotes.
"People have got to know whether or not their president is a crook," Nixon told reporters in November 1973. "Well, I am not a crook."
Since Nixon, every president has released tax returns, except for Gerald Ford, who instead made public a decade's worth of summary data about his federal taxes.
"One of the reasons that Nixon released his returns was that people were saying, 'Hey, how can we trust the IRS to investigate this guy honestly and fairly when he's their boss?'" Thorndike said.
"It underscores the question right now: Can this IRS today do this job? And we have to assume that it can, because there is no backstop. Nixon released his returns in response to these sorts of questions."
Trump's decision not to release returns could end the voluntary practice going forward, Thorndike said.
The decision by presidential candidates is already not cut-and-dried. Some major party candidates have released decades' worth of returns, while others have publicized only the top two pages of their form 1040.
"We may have everyone say, 'Eh, Donald Trump has made tax returns irrelevant,'" Thorndike said. "The tradition is up for grabs right now. It's not clear that it will survive this."
--CNNMoney's Jeanne Sahadi contributed to this report.