Yes, Donald Trump is breaking historical precedent by not releasing his tax returns

Donald Trump has not released his tax returns, making him the only presidential candidate in modern history to refuse to do so.

He has repeatedly said he can't release them because he's under audit by the Internal Revenue Service, something the government agency has said shouldn't bar him from making his tax returns public. 

"As far as my return, I want to file it, except for many years, I've been audited every year," he said during a Republican primary debate in February, according to a transcript by the New York Times. "Twelve years or something like that. Every year they audit me, audit me, audit me.

"I will absolutely give my return, but I'm being audited now for two or three [years' worth] now so I can't."

The IRS released a statement to reporters earlier this year saying there are no restrictions on releasing tax information while being audited.

Donald Trump's tax returns have become a major debate topic. 

During Monday night's presidential debate against Hillary Clinton, Trump repeated the claim he can't release his taxes until the audit is over. "I told you, I will release them as soon as the audit," he said. "Look, I've been under audit almost for 15 years."

He even criticized Mitt Romney in 2012 for not releasing his taxes.

Earlier this month, Donald Trump Jr. said his father couldn't release his taxes because it would detract from his message. "Because he's got a 12,000-page tax return that would create ... financial auditors out of every person in the country asking questions that would distract from [his father's] main message," he told the Pittsburgh Tribune-Review earlier this month when asked why his father hasn't released his tax records.

At the debate, Trump said he would release his taxes if Hillary Clinton "releases her 33,000 e-mails that have been deleted." Then he seemed to say he doesn't even pay taxes.

Clinton: Maybe he doesn't want the American people, all of you watching tonight, to know that he's paid nothing in federal taxes, because the only years that anybody's ever seen were a couple of years when he had to turn them over to state authorities when he was trying to get a casino license, and they showed he didn't pay any federal income tax.

Has Hillary Clinton released her tax returns?

Clinton released her tax returns earlier this month, and after the debate her campaign tweeted about Trump's transparency over his taxes.

Are presidential candidates required to release their tax returns?

While there is no law requiring candidates to release their tax returns, every candidate has done so since 1976, according to PolitiFact.

During the GOP primary, former Florida Gov. Jeb Bush released 33 years of tax returns.

In 1996, Sen. Bob Dole released 29 years of tax returns.

Taxes for Harry Truman and Franklin Roosevelt were released, but not until after their presidency, according to the Tax History Project.

A Senate bill called the Presidential Tax Transparency Act would require candidates to release tax documents within 15 days of being the party nominee.