WITH CHARACTERISTIC incoherence, the White House simultaneously boasted that leaked data from President Trump’s 2005 tax return demonstrates that Mr. Trump paid his fair share of taxes and denounced the “dishonest media” for publishing these flattering facts. But there would be no need for reporters to “illegally” publish such information, to quote the White House’s absurd accusation against MSNBC and investigative journalist David Cay Johnston, if Mr. Trump himself had simply honored the decades-old tradition of releasing his returns while still a candidate for the office he now holds. The bizarre response to the leak only deepens the mystery as to why he still refuses.

To be sure, the new information tends to refute a widely held theory: that Mr. Trump is loath to reveal how little tax he actually pays. In 2005, at least, he paid $38 million, or about a quarter of his taxable income, to the Internal Revenue Service. That’s a higher share than his predecessor atop the GOP ticket, Mitt Romney, reported. No scandal there, though it’s worth noting that this relatively high rate was due almost entirely to the alternative minimum tax, a device for canceling out tax avoidance by the rich that Mr. Trump has proposed eliminating. Keep that in mind when evaluating the White House’s promise that Mr. Trump is focused not on his past returns but on “tax reform that will benefit all Americans.”

Unless and until Mr. Trump releases his returns, including all supporting materials, we are entitled to assume that he has something to hide, whether it’s embarrassingly chintzy charitable contributions, chronic exaggerations of his wealth, business ties to Russia — or some combination of the three. Even tax returns would not constitute sufficient disclosure for a man whose business affairs stretch across states and around the world, as Mr. Trump’s do. The public is entitled to a detailed explanation of all that, as well. It’s true that, shortly before Inauguration Day, Mr. Trump pledged to separate himself from his business empire, which was a step in the right direction. But he did so without specifying the extent of his exposure before he signed away operations to his sons. We have no idea of the status quo prior to Mr. Trump’s claimed recusal.

For the first time in recent history, the presidency belongs to a man with extensive active business investments, creating the potential for conflicts of interest on a correspondingly unprecedented scale. Mr. Trump’s response demonstrates ethical cluelessness at best and contempt for this country’s long-standing democratic norms at worst.

Tax Day is April 18. Mr. Trump has an opportunity to mark the occasion by releasing his Form 1040 for 2016, just as his predecessors made their income tax returns public while serving. If he wants to engender public trust in his leadership, he’ll seize that chance.