All presidents since Gerald Ford have volunteered to show the public their tax returns. All of them except Donald Trump.
He has said emphatically that he really wants to do it, including at a Republican primary debate in February 2016.
"Let me just tell you something. I want to release my tax returns. But I can't release it while I'm under an audit. We're under a routine audit. I've had it for years I get audited. And obviously if I'm being audited I'm not going to release a return. As soon as the audit is done — I love it."
So here's a way around the audit problem: The president can disclose his new return at the same time he files it — before the IRS can start an audit.
Democrats have been pounding on the tax return issue.
"What is he hiding?" asks Bill Pascrell Jr., a Democratic congressman from Paterson, N.J.
Democrats on Capitol Hill have different strategies to force Trump's tax returns into the spotlight.
Pascrell tried last week, but his committee chairman, Texas Republican Kevin Brady, said it's "a clear invasion of privacy."
Pascrell's resolution went down on a party-line vote.
Reporters asked about Trump's 2016 tax filing at Friday's White House press briefing, but spokesman Sean Spicer sidestepped the question and provided no clear answer.
There are multiple reasons presidents should let the public see their tax returns.
Philip Hackney, a former IRS lawyer who now teaches tax law, says there's a higher purpose.
"I think it works more toward an open society, that I think is important," he says. "Democracy."
With Trump, Hackney sees a more pragmatic consideration, too.
"I've never seen a situation where we have someone with the conflicts of interest that he has," Hackney says.
And if the returns were made public, Trump's finances would be clearer — "both domestically, but also internationally," Hackney says.
Speaking of domestic and international, activists plan anti-Trump tax marches on April 15, which falls on a Saturday this year.
"Upwards of 80 marches around the nation, and four marches in Germany, the U.K., Japan and then New Zealand," says Delvone Michael, one of the organizers.
He and fellow marchers will be calling on Trump to disclose his taxes.
The president will have three days to mull it over — Tax Day this year is Tuesday, April 18.