As Donald Trump’s tweets pile one atop another, generating sensational headlines, issues of true substance are tending to get lost in the shuffle. None is more important for voters to keep in mind than the failure of Mr. Trump to disclose his full income tax returns, something he is not likely to do by Election Day.

He is the first major party candidate since 1976 — since Watergate, essentially — to deny voters that vital measure of credibility. It is not required by law that candidates furnish their returns. But Americans have come to expect it.

The interest in Mr. Trump’s case is particularly high. He is running for the White House partly as a business wizard, but is he really as rich and talented as he boasts? Is he as philanthropic as he claims with his reputed billions? Has he truly no conflicts of interest in Russia, whose computer hackers he has bizarrely invited to spy on Hillary Clinton, his campaign rival?

These questions are of Mr. Trump’s own making, and a timely release of his tax returns would provide some answers. “There’s nothing to learn from them,” he tried to insist in May, arguing that he would not make the returns public until after an Internal Revenue Service audit is complete.