Presumptive GOP presidential nominee Donald Trump introduces Gov. Mike Pence (R-Ind.) during a campaign event to announce Pence as his vice-presidential running mate on July 16 in New York. (Evan Vucci/AP)

An earlier version of this story incorrectly identified the pro-Clinton group that orchestrated an offer to give $5 million to charity if Donald Trump releases his tax returns. The group is American Bridge.

Will Indiana Gov. Mike Pence release his recent tax returns?

House Speaker Paul D. Ryan did when he ran as Mitt Romney’s vice-presidential running mate in 2012. Then-Alaska Gov. Sarah Palin did the same in 2008. And then-Sen. Joe Biden released his tax records after he was selected to be the vice-presidential nominee by Barack Obama in 2008 — and has continued to do so annually while serving in office.

Presidential candidates in both parties since Richard Nixon have generally made their tax returns public.

But this year is different, thanks to Donald Trump. Trump has refused to release his returns, citing ongoing Internal Revenue Service audits of his returns that he says would make it unwise to make them public. He told the Associated Press in May that he hoped the audit would be completed soon.

“If it gets finished soon, I put it out immediately, because there’s nothing there. But until you get finished, you won’t,” he said.

Experts, meanwhile, have said there is no technical justification for Trump not to make his returns public, even if he is being audited.

Trump once revealed his income tax returns. They showed he didn’t pay a cent.

The audit excuse is unlikely to be available for Pence, given that the percentage of Americans ever audited is low.

That leaves Trump’s campaign with a conundrum: It will have no ready excuse to hold Pence’s returns back. But it would be highly unusual to release the return for the vice-presidential candidate while withholding it for the top of the ticket.

Trump’s Democratic opponent, Hillary Clinton, is likely to use the vice-presidential selection as a new opportunity to pressure Trump over the issue. She has released all of her and husband Bill Clinton’s returns dating to 1977, and she has needled Trump for not doing the same.

Why Mike Pence said ‘yes’ to Donald Trump

“When you’re running for president and you become the nominee, that’s kind of expected,” Clinton said at a campaign event in May, adding, “You’ve got to ask yourself, why doesn’t he want to release them?”

“Maybe he isn’t as rich as he claims,” she mocked at a June rally. “You have to ask yourself, what’s he afraid of? Maybe that we’ll learn that he hasn’t paid taxes on his huge income? . . . Or that he hasn’t given away as much to charity as he brags about?”

An anonymous Republican donor last week pledged to give $5 million to a veterans charity of Trump’s choosing if he releases his tax returns, a maneuver orchestrated by the pro-Clinton group American Bridge.

Pence’s rivals for the vice-presidential selection would have created different challenges for Trump. New Jersey Gov. Chris Christie had already released his returns, part of an effort to pressure Trump during the Republican primaries. And former House speaker Newt Gingrich had been especially active in pushing then-opponent Romney to release his taxes during the 2012 Republican primary season.

Four times Donald Trump answered Mike Pence’s questions on ‘60 Minutes’

Pence has released some financial information in a disclosure required under Indiana state law; he also filed federal forms for the 10 years he served in Congress. But none of those filings provide the level of detail that would be contained in tax returns.

In particular, tax returns would show Pence’s charitable giving, potentially offering a difficult contrast with Trump. Trump has claimed to have given millions of dollars to charity, but a Washington Post analysis has shown that his actual giving appears to have been far less. Trump has allowed the question to linger by refusing to release tax returns that would probably provide specifics about his giving.

The Trump campaign has almost certainly already reviewed Pence’s returns: In a talk broadcast through Facebook, Gingrich said that the Trump campaign had asked to review his tax returns dating to 2004 as part of the vice-presidential vetting process.