When nominees released their tax returns 6 nominees Reagan Romney Bush ’80 ’12 ’00 200 days before the election 100 50 0 ’88 ’16 ’76 8 nominees Dukakis Clinton Carter 8 nominees 6 nominees 200 days before the election ’88 Dukakis 100 Reagan ’80 Clinton ’16 ’76 Carter 50 Romney ’12 ’00 Bush 0

While not required to do so, all but one major party nominee over the last four decades have released tax returns for the prior year. (Gerald R. Ford released only a summary.)

Last July, Hillary Clinton made public eight years of returns through 2014. She released her 2015 return on Friday.

Donald J. Trump’s position on releasing his returns has shifted over time. Most recently, he has cited Internal Revenue Service audits as a reason for not releasing them.

The Audit Question

The I.R.S. says Mr. Trump is free to release his tax returns despite the audit. There is some precedent: President Richard M. Nixon released his tax returns while he was being audited, though not until after he was re-elected in 1972.

Signing my tax return.... pic.twitter.com/XJfXeaORbU — Donald J. Trump (@realDonaldTrump) October 15, 2015

Robert J. Kovacev, a tax lawyer with Steptoe & Johnson, said he would never advise a client to release taxes under audit. He also points out a major difference between Mr. Trump and Mr. Nixon: “Donald Trump is basically a large corporate enterprise, and that complexity is going to be reflected in his tax returns.”

Joseph J. Thorndike, director of the Tax History Project at the nonpartisan Tax Analysts, says that the complexity of Mr. Trump’s returns is exactly why they should be released. “Unless they are made public, Americans are faced with voting for or against someone whose business interests are almost entirely opaque.”

Why It Matters

Mr. Trump filed financial disclosure forms in May, which is required of presidential candidates. However, experts say tax returns provide more precise data on sources of income, effective tax rates and charitable donations.

Mitt Romney’s returns revealed that he paid an effective tax rate of 14 percent on his income, which came mostly from investments — far lower than the top 35 percent rate for wages and salaries. He was also criticized for holding money in offshore accounts.

“Releasing your tax returns provides voters with a fuller picture into your background, experience, business interests and insight into potential conflicts of interest,” Karen Hobert Flynn of the public interest group Common Cause wrote in a letter to Mr. Trump urging him to release his returns.

Mr. Trump’s Response

It has evolved since he declared his candidacy in June 2015.

“We’ll see what I’m going to do with tax returns. I have no major problem with it, but I may tie them to a release of Hillary’s emails.”

“My returns are extremely complex, and I’ll make a determination at the right time.”

“There’s nothing to learn from them.”

“When the audit ends, I’m going to present them. That should be before the election. I hope it’s before the election.”