One by one, Democratic presidential hopefuls are releasing their tax returns. Years of them. Longtime holdout Sen. Bernie Sanders (I-Vt.) revealed 10 years of returns at the tail end of Tax Day, which arrived on Monday this year. Later that evening, former Texas representative Beto O’Rourke unveiled a decade of his personal tax information. Both followed Sen. Kamala D. Harris (D-Calif.), who released 15 years of tax returns (on April 14), enabling her campaign to boast that Ms. Harris is “the most transparent candidate in the field when it comes to information about her personal finances.”

It is nice to see a race to the top, rather than the bottom, for a change. This is the kind of political one-upmanship Americans should welcome.

Clearly, part of the point is to draw a contrast with President Trump, who routinely promised during the 2016 presidential race to offer his tax information for public scrutiny. He has instead revealed nothing. His excuse is that his returns are under audit, but that would not prevent him from releasing tax documents he signed and swore to be true — and certainly does not explain why he refuses to reveal returns from years ago, as his challengers have done.

The president’s staff has offered nothing more persuasive. Acting White House chief of staff Mick Mulvaney said earlier this month the public would “never” see the president’s tax returns, arguing that the 2016 election had settled the question over whether Americans cared. In fact, Mr. Trump’s persistent promises to disclose more personal financial data remain unfulfilled campaign pledges. White House press secretary Sarah Sanders said (on April 14) that Democrats were not smart enough to understand Mr. Trump’s tax returns, a new defense of Mr. Trump’s indefensible refusal to come clean that hardly rises above the level of a schoolyard insult.

Ever since President Richard M. Nixon set the example, presidents and major candidates have revealed their tax return information to offer voters a view of how these national leaders conduct their private affairs. The returns sometimes expose nothing surprising. Sometimes they result in a headline or two, as when the socialist Mr. Sanders was revealed to be a millionaire. The Post’s James Hohmann pointed out (on April 16) that the most interesting nugget from the Democrats’ recent tax revelations is that many of the candidates have not given much to charity.

Each of these women and men has years, if not decades, of public service that voters can judge. Mr. Trump entered office with a privately held business of unknown value and a Twitter feed. It was all the more important for voters to see how reality matched up to his claims of private business success. Tax returns should have been only the baseline for transparency; a thorough accounting of his business arrangements and possible conflicts of interest was also needed.

It is needed still. Americans remain in the dark about Mr. Trump’s potential conflicts. As Democratic candidates do the right thing, Mr. Trump continues to insult the voters who entrusted him to lead the nation.

-The Washington Post

April 16