When Mitt Romney releases his tax returns Tuesday, the prevailing wisdom is that many middle-class Americans will be upset to see that the Republican presidential candidate paid about 15 percent of his income in taxes: a much lower rate than the typical Joe or Jane collecting a regular paycheck.

But factory workers, cops and bus drivers may not be the only ones unhappy. As it turns out, if his own estimates are accurate, Romney has been enjoying an effective tax rate well below average even among his wealthy peers.

Romney is estimated to be worth more than $250 million, and most of his recent income has come from investments, which are subject to capital gains and dividends taxes of 15 percent. Based on that number, Romney's tax burden is lower than 86 percent of Americans with annual incomes over $200,000, according to Internal Revenue Service data from 2008, the last year for which information is available. There have been no major reforms to the tax code since that year, and most experts believe that the IRS data remains accurate.

Romney's tax liability is even more disproportionately small when one considers how broad an income category more than $200,000 a year is. This group includes everyone from upper-middle-class taxpayers to billionaires. An income of $200,000 puts an individual in the top 6 percent of American taxpayers, but there is nonetheless a big difference between $200,000 and $20 million. It is significant that Romney is in the bottom half of this category in tax liability in spite of being near the very top in income.

Another member of this group, Romney's rival Newt Gingrich, released his 2010 tax returns last week, which only made matters more uncomfortable for Romney. The former speaker of the House paid about $1 million in taxes on $3 million in income: a 32 percent rate.

Romney agreed to release his 2010 returns and an estimate of his 2011 tax liability after suffering a shellacking in the South Carolina primary at the hands of Gingrich. Questions about his tax payments dogged him throughout the campaign in the Palmetto State.

Romney: Low Tax Due to Capital Gains and Dividend Income

Many families earning less than $100,000 a year pay upwards of 27 percent in taxes each year. By his own explanation, Romney is able to pay so little because the bulk of his income comes from capital gains and dividends. Some of his sources of income -- namely speaker's fees and book royalties, estimated at over $300,000 a year -- would be taxed at the maximum rate of 35 percent. But Romney can most likely claim enough itemized deductions to eliminate all that from his taxable income, said Roberton Williams, a tax policy expert with the nonpartisan Urban Institute.

Romney is probably eligible for at least three major deductions, Williams said: charitable contributions (Romney says he gives up to 10 percent of his income to the Mormon church), state taxes (5 percent in Massachusetts) and property taxes on his three homes. If his total income were $10 million, those three deductions alone could reduce his taxable income to $8.5 million.

Warren Buffett, the billionaire investor who has been an outspoken critic of the tax code's preferential treatment of high-income taxpayers, told Bloomberg Television on Monday that Romney's low tax liability was yet more evidence that the American tax code is unfair and out of whack.

He's not going to pay more than the law requires, and I don't fault him for that in the least, Buffett said of Romney. But I do fault a law that allows him and me, earning enormous sums, to pay overall federal taxes at a rate that's about half what the average person in my office pays.