For starters, and assuming that any tax avoidance on his part is legal, we would know exactly what loopholes in the tax code enable someone as wealthy as Mr. Trump claims to be to pay little or no federal tax.

Mr. Trump has attacked loopholes that benefit others, and has called for eliminating the tax treatment of so-called carried interest, which benefits many hedge fund and private equity managers. But has he called for changing any of the provisions that benefit him?

If, as I’ve suggested, he and other real estate developers benefit from their ability to deduct paper losses against ordinary income and to defer capital gains taxes indefinitely through so-called like-kind exchanges, he has proposed nothing that would probably increase his own taxes.

At the very least, he has yet to explain why these loopholes for real estate developers should be retained and how they benefit the public.

It’s also impossible to evaluate Mr. Trump’s broader tax proposals without knowing how they would affect him. Mr. Trump has called for taxing so-called “pass through” income, such as that generated by limited liability companies and partnerships, at a new low 15 percent rate — far lower than the top 33 percent rate he has proposed for individuals. We know from his financial disclosure statements that most of Mr. Trump’s assets are held in the form of L.L.C.s and partnerships. So is this just another self-dealing benefit for Mr. Trump, or is it motivated by the public interest?

There’s no way to tell without seeing his tax returns.

Mr. Trump’s returns would also shed light on how aggressive his accounting has been and how close to the line of legality he has been willing to walk. As Steven M. Rosenthal, a tax lawyer and senior fellow at the Urban-Brookings Tax Policy Center, recently put it, “Did Trump lower his taxes through legal tax avoidance, or illegal tax evasion?”

Mr. Trump has never been charged with any wrongdoing, but there’s a large gap between illegal behavior that can be proved beyond a reasonable doubt and what most people would consider appropriate conduct by a candidate for president. What Mr. Trump considers “smart” might be seen by others as evasive. “Candidate Trump should follow the precedent set by every major party presidential candidate — and president — since Nixon and release his tax returns,” Mr. Rosenthal said.