That move will occasion a response from Neal -- in the form of a subpoena, a contempt vote or a lawsuit. No matter what Neal chooses, this fight is headed into the legal arena sooner rather than later.

And it's likely that it winds up, at some point, in front of the Supreme Court.

Which is, if you stop and think about it, remarkable. We are now using the Supreme Court to litigate something that, prior to Trump, was seen as de rigeur for people running for president. Sure, some candidates released more years of their tax returns. And some released less. Some did so with little prompting while others -- Bernie Sanders in 2016 -- had to be dragged kicking and screaming to transparency.

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But, the point remains: Donald Trump was (and is) the only major party presidential nominee since Watergate to never release any past tax returns. He is also the only president to refuse to release even a year of past returns.

So, why? Why never release any tax returns during the course of the campaign? Why offer a variety of shifting explanations -- under audit, too complex, no one cares -- for why you aren't releasing them? And why carry the fight possibly all the way to the nation's highest court?

Mnuchin's stated reason for not turning over the taxes to Neal is that the Massachusetts Democrat is not asking for the returns for legitimate reasons but rather to score political points -- and, therefore, the Internal Revenue Service (and the Treasury) doesn't need to comply with a little-known statute that allows a select few members of Congress to see Trump's tax returns.

"In reliance on the advice of the Department of Justice, I have determined that the Committee's request lacks a legitimate legislative purpose, and ... the Department is therefore not authorized to disclose the requested returns and return information," Mnuchin wrote to Neal on Monday.

No matter where you come down on that legal technicality, it still doesn't explain Trump's long-running resistance to releasing his tax returns. He's been fighting against transparency on this front from the beginning of his candidacy -- despite running a campaign in which he put his supposed wealth and success front and center.

Which gets me back to the question of why. Here are a few of the most likely reasons:

1) Trump's not as rich as he says: There's very little question that Trump overstates his personal wealth -- and has done so for as long as he has had personal wealth. He's said he's worth $10 billion; : There's very little question that Trump overstates his personal wealth -- and has done so for as long as he has had personal wealth. He's said he's worth $10 billion; Forbes said in 2018 that number is closer to $3 billion. For Trump to be exposed as one-third as rich as he had said he is would be embarrassing, sure. But remember, too, that Trump equates wealth directly with success and importance. So being exposed as less rich would be a blow to his success narrative as well.

2) He has Russia ties: While the Mueller report made clear that neither Trump nor anyone in his campaign orbit colluded with the Russians to help Trump win the 2016 election, it would still be a very big story if Trump's tax returns showed he had extensive financial dealings and ties to Russia. We know, from Michael Cohen, that Trump and his family were deeply involved in the potential construction of Trump Tower Moscow and that Cohen lied about that involvement (and how long it stretched) to protect Trump. We also know that Donald Trump Jr. : While the Mueller report made clear that neither Trump nor anyone in his campaign orbit colluded with the Russians to help Trump win the 2016 election, it would still be a very big story if Trump's tax returns showed he had extensive financial dealings and ties to Russia. We know, from Michael Cohen, that Trump and his family were deeply involved in the potential construction of Trump Tower Moscow and that Cohen lied about that involvement (and how long it stretched) to protect Trump. We also know that Donald Trump Jr. said this at a 2008 real estate conference: "In terms of high-end product influx into the US, Russians make up a pretty disproportionate cross-section of a lot of our assets. Say, in Dubai, and certainly with our project in SoHo, and anywhere in New York. We see a lot of money pouring in from Russia."

3) He didn't pay many (or any) taxes: We know -- : We know -- because Trump admitted as much -- that he used a more than $900 million net operating loss for his business in 1995 to avoid paying personal income taxes for a number of subsequent years. How? This, from The New York Times , explains it:

"Known as net operating loss, it allows an array of deductions, business expenses, real estate depreciation, losses from the sale of business assets and even operating losses to flow from the balance sheets of those partnerships, limited liability companies and S corporations onto the personal tax returns of people like Mr. Trump. In turn, those losses can be used to cancel out an equivalent amount of taxable income."

As the Times noted, Trump's $916 million loss in 1995 could have allowed him to pay $0 in personal income taxes for the next 18 years.

4) He donates no (or very little) money to charity: We know, thanks to the amazing reporting of the Washington Post's David Fahrenthold, that Trump long used his charitable organization to feather his own nest and collect political chits rather than for any philanthropic purposes. ( : We know, thanks to the amazing reporting of the Washington Post's David Fahrenthold, that Trump long used his charitable organization to feather his own nest and collect political chits rather than for any philanthropic purposes. ( Trump shut down the charity in 2018 .) It's not at all clear how generous (if at all) Trump has been to other charities over the past few decades. While there is no requirement for wealthy individuals to make large charitable donations, many do. And Trump has often boasted about the depth and breadth of his charitable givin g

Whatever the reason (or reasons), it's very clear that Trump -- soon after he became a candidate for president -- reversed course on his past pledges to release his returns.

"I would certainly show tax returns if it was necessary," Trump told conservative radio host Hugh Hewitt in February 2015 , just months before formally entering the presidential race. A year later, after Mitt Romney publicly challenged Trump to release his taxes, the billionaire businessman's approach had changed.

"You don't learn anything from a tax return," Trump said at a GOP debate in February 2016. "I will say this. Mitt Romney looked like a fool when he delayed and delayed and delayed and Harry Reid baited him and Mitt Romney didn't file until a month and a half before the election and it cost him big league ... As far as my return, I want to file it except for many years, I've been audited every year. Twelve years or something like that. Every year they audit me, audit me, audit me ... I will absolutely give my return but I'm being audited now for two or three [years' worth] now so I can't."

(Nota bene: There is no law that forbids a president -- or any citizen -- from releasing their taxes while under audit.)

What changed? Who knows! But, it's quite clear that sometime between February 2015 and February 2016, Trump decided that whatever was in his returns would be far worse for him if it got out than the negative press he would take for not releasing them. And he's not backing away from that decision. Not for anything.