Mr. Trump has no record of military service. He has never held elected office. Born wealthy, he took over his father’s business and built a spotty track record. Disclosing his returns might enable Mr. Trump to support one of his main claims on the presidency: that he’s a negotiator so skilled it has made him a billionaire.

Yet Mr. Trump has a history of becoming irate when the subject of his income taxes comes up and belligerent when journalists have caught him misrepresenting his income and charitable contributions. A decade ago Mr. Trump sued one of them, Timothy O’Brien, an editor at Bloomberg View, for libel.

Mr. Trump lost, but in the course of the suit, he was ordered to provide his tax returns. He delayed for months, then produced documents so redacted with black marker that they looked “like crossword puzzles,” Mr. O’Brien wrote this week. Mr. Trump finally produced more readable returns, but Mr. O’Brien said he was forbidden by court order to discuss specifics.

Though Hillary Clinton continues to keep the contents of her Wall Street speeches under wraps, she has, to her credit, released years of tax returns. And Mr. Trump, in the past, has been a stalwart advocate of disclosure. On Friday the Democratic National Committee released a video of his televised promises, dating back years, to release his returns. The video includes an interview in which he notes that “everybody has done it for many, many years.”

Mr. Trump now says he won’t release his returns because he’s being audited. Such concern didn’t stop President Nixon from releasing several years of returns in 1973 — even though the Internal Revenue Service subsequently determined that the president owed nearly $500,000 in back taxes. (Mr. Nixon’s famous comment, “I’m not a crook,” didn’t refer to Watergate, but to rumors about tax avoidance, which turned out to be accurate.)