AS REPUBLICAN voters from Maryland to Rhode Island go to the polls Tuesday, there is a lot they will not know about Donald Trump. Unlike his rivals, he has no history in public office on which to judge his suitability for political leadership. This has enabled the billionaire to attack politicians, who have to defend political records open for everyone to see, while touting his business record, which is not subject to a similar level of transparency. His business interests are often private, and his required financial disclosure reports aren’t vetted for accuracy, leaving him free to make wild claims about his success.

Mr. Trump’s unusual position means that the scrutiny that routinely accompanies running for president is all the more important in his case, starting with the release of his tax returns. Yet Mr. Trump also has been the least transparent candidate. His GOP rivals have disclosed tax returns going back at least four years. Meanwhile, the real estate developer has stalled, puzzlingly declaring that his tax returns are “very beautiful” while offering laughable excuses for refusing to share them with the public.

Mr. Trump’s primary defense is that the Internal Revenue Service is auditing his tax submissions. This presents no obstacle to him releasing earlier returns. There is also nothing stopping Mr. Trump from disclosing his preliminary tax documents even while the government is reviewing them. The differences pre- and post-audit could be illuminating. So could many other details. Maybe the returns would provide evidence that Mr. Trump’s business dealings are not generating as much profit as one might expect. Perhaps they would demonstrate that he does not give much to charity, as reporting from The Post’s David A. Fahrenthold and Rosalind S. Helderman suggests. Maybe there would be other surprises.

Mr. Trump claims that tax returns do not show all that much. This argues for releasing more information, not less. Presidential candidates have in the past gone beyond releasing personal tax returns. Mitt Romney, for example, disclosed tax information from the Tyler Charitable Foundation, the entity that handles his charitable giving, during the 2012 campaign. True, information on Mr. Romney’s generosity tended to paint him in a positive light. But if the story Mr. Trump has told voters is true, shouldn’t releasing information about his business dealings help him? The GOP front-runner has made his business prowess central to his campaign. Along with his personal tax returns, he should release more hard information about the performance of his private ventures so that voters can judge whether he is the business genius he claims to be.

Mr. Trump is not the only candidate who has struggled with transparency. It took Sen. Bernie Sanders (I-Vt.) a strangely long amount of time to finally release a single year’s tax return. Hillary Clinton has yet to reveal the transcripts of the speeches she was paid to give after leaving the State Department. But Mr. Trump is in a stonewalling class of his own.