Romney, the frontrunner for the Republican nomination to face Democratic President Barack Obama on November 6 and a former private equity executive with an estimated net worth of $270 million, has been reluctant to lift a curtain on his vast financial holdings.
In recent days, Romney's increasingly desperate rivals - namely former House of Representatives speaker Newt Gingrich and Texas Governor Rick Perry - repeatedly have questioned whether Romney, in not releasing the tax returns, is hiding something.
Their calls for Romney to release his returns were echoed on Tuesday in a New York Times editorial, which called Romney's "insistence on secrecy impossible to defend now that he appears to be closing in on the nomination and questions have intensified about his personal finances."
During Monday's Republican presidential candidate debate in Myrtle Beach, Romney said, "I have nothing in (the returns) that suggests there's any problem and I'm happy to" release them around the federal tax deadline in mid-April.
"I sort of feel like we are showing a lot of exposure at this point," Romney added. "And if I become our nominee, and what's happened (with past presidential candidates) is people have released them in about April of the coming year, and that's probably what I would do."
FORTUNE INVESTED IN BAIN FUNDS
Tax analysts say Romney, one of the wealthiest presidential candidates in U.S. history, may have good reason to be reluctant to release his returns.
His vast fortune is invested in dozens of funds linked to Bain Capital, the powerhouse private equity firm he led for 15 years. Several Bain funds have offshore connections and take advantage of tax breaks used only by the U.S. financial elite.
His tax returns could shed light on how Romney and Bain use offshore strategies to avoid taxes, said Daniel Berman, a former U.S. Treasury deputy international tax counsel and now director of tax at Boston University's graduate tax program.
Bain funds in which Romney is involved are scattered from Delaware to the Cayman Islands and Bermuda, Ireland and Hong Kong, according to a Reuters analysis of securities filings.
"Certain interests in foreign investment structures would have to be reported on attachments to his return," Berman said.
The wealthiest Americans typically earn a large chunk of their income from investments - much of it in capital gains.
Because capital gains generally are taxed at 15 percent compared with the top ordinary income tax rate of 35 percent, those with significant income from capital gains may pay lower tax rates than many Americans.
On capital gains, Romney's tax returns would not reveal any gains that he has not yet realized, even though those gains would be easy for him to lock in at any time, Berman said.
"I remember as a young lawyer being surprised to see tax returns of very successful investors showing net losses - because they were recognizing net losses - not unrealized gains," Berman said.
Romney's returns also might not spell out how much he benefits from a tax break called the carried interest loophole.
This rule allows private equity and hedge fund managers to pay the 15 percent capital gains tax rate, rather than the top income tax rate, on a large portion of their earnings.
After a campaign event on Tuesday in Florence, South Carolina, Romney told reporters that his tax rate is "probably closer to 15 percent than anything."
Romney also said he gets speaker fees "from time to time, but not very much."
Annual campaign financial disclosure forms indicate that Romney was paid more than $374,000 in speaker fees from February 2010 to February 2011.
A SERIES OF ATTACKS
The demands by Gingrich and Perry are their latest attempt to draw attention to Romney's wealth and portray him as out of touch with the concerns of most Americans.
They also echo Gingrich and Perry's criticism of Romney's time at Bain, which Romney co-founded in 1984 and left in 1999. Bain was involved in overhauling dozens of companies, and in some cases laid off thousands of workers.
Gingrich, Perry and others have portrayed Romney as a job killer and, as Perry put it, a "vulture capitalist."
The attacks don't seem to have worked. Romney is still riding high in public opinion polls of the Republican candidates. And some conservative Republicans accuse Gingrich and Perry essentially of an assault on capitalism.
Gingrich and Perry went after Romney on the tax return issue during Monday's debate, and Gingrich continued to pound on the theme Tuesday.
"It's interesting that Romney agreed that he ought to release his income taxes but he doesn't want to do it until April," by which time Romney could have clinched the Republican nomination, Gingrich said during an interview with CBS.
"I think the people of South Carolina ought to know now -- if there's nothing there, why hide it until April? And if there's something there, don't the people of South Carolina deserve to know before Saturday?"
Gingrich added that he would release his tax returns this week. As Texas governor, Perry has released his each year.
Gingrich and Perry are battling former Pennsylvania U.S. senator Rick Santorum to put together enough conservative votes to block Romney's march to the nomination.
Romney won the Iowa caucuses and New Hampshire primary this month - the first two nomination contests - and is favored to win the South Carolina primary Saturday as well as Florida's primary on January 31.
Santorum, thought earlier this month to be Romney's main challenger, has not been as vocal in calls for Romney to release his tax returns.
A Santorum aide said that he was unsure whether Santorum would press Romney on the matter, but said, "We've been a pretty staunch advocate of airing out all the laundry now."
"We don't need any surprises," the aide said. "We need to know now."
The Romney campaign dismissed the latest calls to release his tax returns as a sign of desperation.
"This is pasta politics," Eric Fehrnstrom, a senior Romney adviser, said. Gingrich is "throwing spaghetti against the wall to see what sticks."
(Additional reporting by Kim Dixon and Kevin Drawbaugh; Editing by David Lindsey and Will Dunham)