MELANIA TRUMP’S Monday night speech to the Republican National Convention was so platitudinous — a gauzy collection of assurances that her husband, Donald Trump, is “kind and fair and caring” and will “never, ever, let you down” — it could have been delivered by any spouse about any candidate. In fact, as has been widely reported, some of it had been delivered by a different spouse about a different candidate.

No matter. The information Americans lack about Mr. Trump concerns not his personality, which is all too evident, but his life’s work before entering politics — his business record and tax history. Mr. Trump constantly assures Americans that he built a “great company” and is worth $10 billion. He promises to bring his no-nonsense business sense to government, renegotiating trade deals, striking bargains with drug companies and streamlining the federal government. His gilded private jet has been a campaign prop. Yet his company, the Trump Organization, is a closely held private enterprise, with many of its dealings hidden from public view. Bits of information that have emerged suggest Mr. Trump has had some success licensing his name and starring on reality TV. But a variety of Mr. Trump’s business ventures, such as Trump University, appear to have been unethical, unprofitable or both. The story of his professional life is unacceptably incomplete.

Mr. Trump released a candidate financial disclosure form months ago. But it was thin on detail and, possibly, long on exaggerations of his wealth. He still has not released his “beautiful” tax returns despite promising to do so. Breaking a public commitment would be unsurprising for Mr. Trump. But refusing to release his tax returns would also break a long-standing bipartisan tradition. Every major-party presidential nominee submits to thorough public vetting, including of their private business dealings. This is even more important in Mr. Trump’s case because he has no record in public office on which voters can judge his suitability for higher office.

Mr. Trump has offered various excuses for breaking his promise of disclosure, often citing an ongoing Internal Revenue Service audit of his returns. But that does not prevent him from releasing those returns, which he swore to the government were accurate when he submitted them. Nor would it prevent him from releasing past returns.

Theories abound about the true motivation for his recalcitrance. Maybe Mr. Trump’s reported earnings would show he is not as successful as he has claimed. Maybe we’d learn that he has given little or no money to charity in recent years. Maybe embarrassing foreign tax shelters would be revealed. What is certain is that useful information would emerge.

Mr. Trump is evading transparency and eroding one of the country’s essential democratic norms. As his family seeks to reintroduce him to the nation with a series of testimonials this week, Mr. Trump should do his part and keep his word: Release the tax returns.