With President Obama's campaign hammering Mitt Romney over his personal finances, even some Republicans have begun calling for the presumptive GOP nominee to release more than two years of his tax returns. Conservative political analysts say Romney is allowing people to assume he has something to hide. "It's crazy" not to release several more years of returns, "tomorrow," says William Kristol, editor of The Weekly Standard. Despite the pressure, Romney said Monday on Fox News that he was releasing his 2010 and 2011 returns, and nothing more. Why won't he turn over documents from other years? Here, six theories:

1. Romney's income might have been extremely high

Romney earned $21.7 million in 2010 and $20.9 million in 2011, says John Cassidy at The New Yorker. That's a lot of money, "but Romney may well have earned considerably more in earlier years." In fact, Romney's 10-year severance agreement with Bain Capital ended in 2009, so going back just one more year might tell a lot about how the mysterious deal "allowed Romney to keep pocketing a substantial portion of the firm's profits. And the years before 2008 were massively successful ones for Bain and other private-equity firms."

2. His tax rate might have been close to zero

Romney might have done "something that was kind of shady — not in an illegal sense"—to bring his tax rate way down, Martin Sullivan of Tax Analysts tells Agence France-Presse. Since Romney's income in the return he has already released from 2010 came from investments, not income, it was taxed at 15 percent, less than half what he would have paid for the same income in a paycheck. Let's say he got his effective tax rate to five percent or even to zero. "That would be damning," as it would reinforce Team Obama's efforts to portray Romney as rich and "out of touch."

3. Romney might actually have something to hide

As conservative commentator George Will and others have pointed out, Romney knows withholding his returns is hurting his campaign, says Daniel Shaviro at Start Making Sense, so it's logical to assume that whatever is in the returns is worse. Remember all the talk about Romney's Swiss bank account? "There was an IRS amnesty program in 2009 for fraudulent nondisclosure of offshore income." If Romney came clean that year, "this might be embarrassing." One thing we do know: Romney's 2010 return showed a net capital loss carryforward from 2009, the year when his last windfall of deferred interest came from his "retroactive retirement" from Bain. This suggests Romney might have done some "extremely aggressive tax sheltering" in 2009 that voters might frown on.

4. Each return provides a reminder of Romney's Mormonism

Romney's tax returns "reveal the extent of his tithing to the Church of Jesus Christ of Latter-day Saints," says Doug Mataconis at Outside the Beltway. His 2010 return, for example, showed that he had given roughly $4.1 million to the LDS Church, all of it tax deductible. Letting the public see year after year of records would "emphasize Romney's Mormon ties at a time the campaign might not want to do so."

5. Romney is just being arrogant

"There is nothing in those tax returns that is in any way illegal," says Charles P. Pierce at Esquire. Romney's high income and low tax rate won't tell us anything we don't know already. It's no secret that "our tax code — and, indeed, our entire economic system — has been gamed to benefit the folks in Romney's economic stratum." Romney merely has contempt for a process that requires someone of his wealth and privilege to show deference to people who have neither. "The Help" — that's the rest of us — "has no right to go pawing through the family books."

6. He won't give in to attempts to divert attention from Obama

There's a perfectly good reason for Romney to refuse to release his tax returns, says Bob Webster at WEBCommentary: They "are none of anyone's business." The U.S. tax code is a mess, and "antagonistic lawyers" could pick apart the personal returns of anyone at Romney's income level. It would be "insane" for Romney to give in. "The whole non-issue is designed to be part of a smokescreen to cover up the gross ineptitude" of President Obama during his four disastrous years in office.

Read more political coverage at The Week's 2012 Election Center.