Kellyanne Conway, counselor to Mr. Trump and his chief obfuscator, told ABC News on Sunday that “he’s not going to release his tax returns,” adding that the election showed that “people didn’t care.” On Monday, she pulled back a tad, tweeting that “POTUS is under audit and will not release until that is completed.” Of course, even that comment is a ruse. The Internal Revenue Service has made clear that being under audit wouldn’t preclude Mr. Trump from making his returns public.

Yet, the Trump campaign used that excuse over and over, and now Mr. Trump has carried it into the White House. White House officials are probably hoping that the longer they stonewall, the more likely that public demands on this matter will be pushed aside as torrents of controversial policies and statements from Mr. Trump dominate the news cycle. Even so, voters and members of Congress who care about ethics in the nation’s highest office should not let up.

Releasing the returns would provide important insight into Mr. Trump’s finances and businesses. They would reveal if he is as wealthy as he claims to be, what his effective income tax rate is (he said during the campaign that not paying taxes meant he was smart) and how much he gives to charity. The documents would also identify the sources of his income and debt, helping to answer questions about his links to businessmen, banks and governments in places like Russia and the Middle East, and putting a spotlight on potential conflicts of interest.

Presidential candidates have voluntarily disclosed their tax returns since the Watergate scandal ushered in an era of greater transparency. Mr. Trump, whose checkered past as a businessman includes a string of bankruptcies and a $25 million settlement compensating students who said they had been defrauded by Trump University, has chosen to buck this trend, perhaps because he has something to hide.

Congress can force his hand by supporting a bill that Senators Ron Wyden of Oregon and Chris Murphy of Connecticut introduced this month. It would require the current and all future presidents to release their tax returns. State lawmakers could also head off this problem in the future by forcing presidential candidates to disclose their tax returns to get on the ballot. There is one such bill pending in New York.