President Trump in Washington. (Bill O'Leary/The Washington Post)

AS MANY Americans scramble to file their tax returns by Tuesday’s deadline, one is brazenly defying expectations regarding his tax information, making 2018 the third year in a row he will have gotten away with it. When he was a candidate, Donald Trump repeatedly promised to let the public see his tax forms. He has yet to do so.

For most Americans, financial records are rightly private. For every president since Richard Nixon — and even for major presidential candidates — the standard has been different, and for good reason. Mr. Nixon disclosed his returns amid questions of whether he skimped on his payments to the Treasury. He had, investigators concluded after he turned them over. The public could not have confidence that the Internal Revenue Service would adequately police the president, the agency’s boss, without public review. President Trump’s sprawling, secretive business entanglements, from which he refused to divest, makes this problem more acute. Mr. Trump complains that his tax returns have been subject to audit — one of the many bogus excuses he has given for keeping them hidden. In fact, he is free to release them even if an audit is taking place. He could certainly also release returns from past years.

The way presidents conduct their private business affairs reflects values they may apply to their public duties — and it reveals conflicts of interest they bring to the office. Again, because Mr. Trump maintains a wide-ranging private business, with relationships that the public can otherwise only guess about, releasing his returns is more, not less, urgent than for most presidents. If Mr. Trump intends to act in the public interest rather than his own, he should have nothing to fear from full transparency. That would require not just releasing tax returns but also providing a detailed record of the Trump Organization’s business relationships.

Instead, the president is setting a precedent — that presidents can promise one thing, do another and end up dismissing essential standards of disclosure. Congress should not accept this erosion of good-government practice. Federal lawmakers should require major presidential candidates to release their current and past tax forms. If Congress does not do so, state legislatures could make disclosure mandatory for candidates seeking to appear on the ballots within their borders. Another possibility worth considering is for states that already have the president’s tax information, such as New York, to release the records they have kept.

One way or another, Americans should not accept Mr. Trump’s contempt for them that is implicit in his broken promise, nor his contempt for the norms that promote trust in government.