Over the past 40 years, nearly every major presidential candidate has released his or her tax returns to the public in their pursuit of the White House. It's a way to prove what their income is, how much they give to charity and whether they have investments. Though publishing the documents isn't legally required, it's become tradition — one that Republican nominee Donald Trump may be trying to break.

The billionaire real estate developer has repeatedly refused to reveal his tax returns — at least while he's being audited by the Internal Revenue Service. In the meantime, he told ABC News last week, "I don't think anybody cares, except some members of the press."

But pressure has been ramping up for him to publicize the documents soon. New York Times executive editor Dean Baquet and Washington Post associate editor Bob Woodward said Sunday they'd report on Trump's tax returns even if they went to jail for it, according to CNN. Veteran Peter Kiernan started a $25,000 crowdfunding campaign to be donated to military charities once the GOP candidate releases his taxes, and LinkedIn cofounder Reid Hoffman said Monday he'd contribute five times the amount raised (up to $5 million).

You may be wondering why it's a big deal. Here are six factors that explain the recent buzz around Trump's taxes:

He has a lot of money (probably).

Trump is, as he likes to say, "really rich." In May, he claimed he was worth more than $10 billion and filed a financial disclosure form showing he earned $557 million between then and January 2015, according to a news release. But that didn't take "dividends, interest, capital gains, rents and royalties" into account.

Releasing his taxes would show what his tax rate is as well as what business relationships he has, CNN reported. That might speak to his loyalties as a president.

The IRS isn't stopping him.

The agency told the Huffington Post in February that, contrary to Trump's claims, "nothing prevents individuals from sharing their own tax information." Several media outlets have noted that former President Richard Nixon released his taxes while being audited in the 1970s.

Trump criticized President Barack Obama in 2012 for a lack of transparency.

Four years ago, Trump offered to donate $5 million to charity if Obama made his college transcripts and passport records public by a certain date. At the time, he declared Obama "the least transparent president in the history of this country," Politico reported.

Trump's charitable gifts are hard to track.

When he was Trump's campaign manager, Corey Lewandowski told CNN the mogul had donated more than $100 million to charitable causes. But Washington Post reporter David Fahrenthold recently called more than 300 charities linked to Trump and discovered the candidate had only personally funded one gift between 2008 and May 2016. He hasn't donated to the Donald J. Trump Foundation in eight years, CNN confirmed.

His rival has already released hers.

Democrat Hillary Clinton published her 2015 tax return in August, showing $10.6 million in income. Her website notes that, between various campaigns for her and her husband, former president Bill Clinton, their taxes have been public since 1977.

People want to see them.

It's not just pundits and journalists clamoring for Trump's taxes: In a Morning Consult poll from May, about two-thirds of voters said they thought presidential candidates should have to reveal their tax returns. That's given Clinton ammunition to call for the release of the documents. 

"The burden is on him, and indeed for the rest of us in the campaign on our side, the press, the public, to demand what a big majority of the public says they want," USA Today reported she said Sept. 6.