What did Donald Trump do today?

He officially refused to let Congress examine his tax returns.

Last month, the chair of the House Ways and Means Committee, Rep. Ed Neal (D-MA), formally instructed the IRS to release Trump's personal and business tax returns for that committee to examine. Neal was empowered to do this by a 1924 law which states that when certain committee chairs request tax returns, 

the Secretary [of the Treasury] shall furnish such committee with any return or return information specified in such request, except that any return or return information which can be associated with, or otherwise identify, directly or indirectly, a particular taxpayer shall be furnished to such committee only when sitting in closed executive session unless such taxpayer otherwise consents in writing to such disclosure.

Today, Trump's Treasury Secretary Steven Mnuchin refused to obey the law, saying that he had determined that Congress "lacks a legitimate legislative purpose" in asking for the returns. Mnuchin also said that he was "not authorized to disclose" Trump's tax information. (The law itself is the "authorization.")

Importantly, the Ways and Means Committee doesn't need Trump to approve of its "legislative purpose."

The more interesting question may be why Trump has, unlike every president and major-party candidate since President Nixon, is so afraid of even a closed-door examination of his private finances.

It's sometimes suggested that Trump is simply embarrassed by the fact that he's not as wealthy as he pretends to be. Psychologically, that's plausible: Trump was once caught on tape impersonating his own (imaginary) press agent as part of a scheme to fake his way on to the Fortune 400 list.

Another theory is that Trump's tax returns would make clear how deeply in (monetary) debt he is to foreign banks, in particular Russian ones. It's not illegal for a president to owe money—but hiding that fact while pursuing radically pro-Russian policies would cripple Trump politically.

But it's most likely that Trump is simply afraid of crimes coming to light. Unlike most ultra-wealthy heirs, who use legal means to reduce their tax bills, Trump's father gave him the equivalent of $400 million while conspiring with other family members to illegally evade gift and inheritance taxes. His business tax returns could also shed light on the extent to which he was complicit in money laundering for Russian oligarchs and organized crime figures. Trump is also vulnerable to charges that he defrauded banks or investors by lying about the value of his assets, and his tax returns would shed light on that, too.

Why should I care about this?

• Presidents are not above the law.

• People suspected of crimes don't get to decide whether or not they're investigated.

• It's bad if the best possible explanation for a president's behavior is "he's ashamed of not really being a billionaire."