Hillary Clinton released her 2015 tax return on Friday. It wasn't an earth-shattering event. Everyone expected her to do so, because everyone expects this of all presidential candidates.

Or at least, everyone expected it before this year.

Because Clinton has released tax returns frequently in the past, hers contain relatively little that wasn't already known. She and her husband, former President Bill Clinton, make millions from speeches and book royalties, and donate to their own foundation, the one whose donors can evidently purchase State Department access and possibly even positions in government.

Clinton has repeatedly lied to voters about her conduct in office and in the time since. She hid tens of thousands of work emails from the government and taxpayers for years, then pretended to release all of them when she really hadn't.

So the idea that she could somehow seize the high ground on the issue of transparency is as ridiculous as it is galling. Yet Donald Trump has allowed that to happen because he refuses to release his own tax returns. It suggests he is hiding something, even if it is only that he is not nearly as rich and successful as he claims.

There are some norms and standards that presidential candidates can afford to shirk. They don't have to be politically correct or take positions that ingratiate them to East Coast elites. They don't have to stay on their party's message or hew to their party's policy line to be taken seriously. They don't even need more than a rudimentary understanding of most political issues.

But even the most unconventional presidential nominee in decades cannot hide who he really is. All other candidates release their tax returns, and it's why Trump should do so, too, and without delay.

Trump repeatedly claims on the campaign trail that he cannot be bought and that he is very rich. The more he refuses to reveal the details of his finances, the harder it is to believe anything he claims about them.

Not only is there no good excuse for concealment, but the excuses he offers are fundamentally unserious. Even if he is being audited, a claim he has made that cannot legally be checked, it does not prevent him from releasing his returns. Even if it did, it would not prevent him from releasing returns from earlier years, as Mitt Romney did in 2012.

With so many obstacles standing between Trump and the presidency — a hostile news media, his record unfavorable poll ratings, his mediocre fundraising, all come instantly to mind — the problem created by his refusal to be transparent about his tax returns is a problem for which no one can be blamed but himself.

If he can't figure that out himself, it is difficult to know how his candidacy can be helped.