Washington (CNN) President Donald Trump is breaking with modern precedent in his refusal to succumb to pressure to publicly release more information on his past tax payments on the day that every American is expected to privately file theirs.

For the past 40 years, every US president and presidential candidate has released tax documents. In 1973, while under audit and amid a controversy over his personal taxes, Richard Nixon released returns dating back to when he took office in 1969.

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.

For the US president and vice president, an audit is guaranteed every year.

The practice of doing a "mandatory examination" of the presidential and vice presidential tax returns has been in the Internal Revenue Manual since the Watergate era, according to the IRS.

Trump first abandoned modern precedent by not releasing his tax returns during the presidential campaign, citing an ongoing routine audit. And as of Tax Day, he has still not done so -- and is, in fact, doubling down and signaling that he could never release them.

"We'll have to get back to you on that," White House press secretary Sean Spicer said Tuesday when asked if the White House would declare that the returns would never be public, even after the completion of the audit.

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Spicer and Trump have argued that voters declared the issue moribund, electing him even though he shielded the returns. But the controversy could be reignited with the White House's coming push for tax reform, with some liberals eyeing a package that forces the IRS to disclose the returns, though Republicans would certainly block it.

Nevertheless, Democrats this weekend staged a series of marches in 200 cities calling for their release.

Sen. Elizabeth Warren of Massachusetts said Tuesday morning that the issue was not dead.

"The issue is not over. He promised during the campaign that he would reveal his taxes. In fact -- how many clips have you got here?" she said on NBC's "Today." "He would reveal them after this. He would reveal them after that. Oh, then he would put it off."

"I think that people are going to keep demanding it," she added.

Freshman Republican Rep. Matt Gaetz of Florida told a town hall in February, that the President "absolutely" should release them. But most Republicans have not made similar calls, and some of them have recently been under fire for defending Trump's position at the time of the year when constituents are paying their taxes.

Tom Cotton, a Republican senator from Arkansas, said Monday in Little Rock that Trump had already disclosed some financial data and that "it doesn't take a lot of effort" to figure out where Trump has investments and foreign connections.

The crowd booed.