Still, critics are questioning whether Romney went too far in deferring or avoiding taxes by his use of an IRA, noting that Congress has put limits on contributions to prevent too much income from being shielded from taxation. Under current law, individuals typically are limited to contributing $5,000 per year, and companies with a type of IRA known as a Simplified Employee Pension Plan are limited to contributing $50,000 annually to their employees’ accounts. The latter plan is similar to the one used at Bain.

The short-term capital gain profits “could have been taxed at 40 percent and were instead taxed at zero,” one Romney associate said, referring to tax rates in effect at the time of the 1996 Experian deal. In many other cases, where longer-term gains could have been taxed at a lower rate, they similarly would not have been subject to a tax until the funds were withdrawn. The cumulative effect — with profits from each success reinvested in the next deal — helps explain how Romney’s IRA could have grown so large, according to his associates.

Or exploded, in some cases. For example, in one deal for a credit-reporting service known as Experian, Bain tripled its $100 million investment in just seven weeks. This could have resulted in a 40 percent tax on short-term gains for partners who made personal investments in that deal. But by using funds within an IRA to make the investment, Bain partners would not have to pay any tax on their personal stake until the funds are withdrawn upon retirement. While Romney has not said whether he used the IRA in that particular deal, his associates said the practice was widely used at Bain.

Romney has not provided details about how his IRA grew so large. But Romney associates with direct knowledge about the matter said Bain Capital partners used their IRAs as a pool of investment money, enabling them to make personal investments in Bain deals, many of which earned spectacular returns. Much as a lower-dollar investor might pick mutual funds for an IRA, the Bain partners could make side investments in the firm’s deals and then watch as their retirement funds grew.

Even the lesser amount would put Romney’s IRA in the top 0.001 percent of all such accounts in the country, according to analysis by the nonpartisan Employee Benefit Research Institute. The extraordinary size of Romney’s IRA has led Democrats to question how he could amass such a fortune given that annual contributions to the fund are strictly limited.

It is one of the most striking elements of Mitt Romney’s financial fortune. He has used the seemingly bland investment vehicle known as an individual retirement account — established by Congress to help average Americans save a modest amount for retirement — to shield at least $20 million and as much as $100 million from initial taxes.

The extent of Romney’s tax deferral or avoidance has become an issue in the presidential campaign. Democrats have charged that Romney used an array of tactics, including the IRA and offshore vehicles, to avoid huge tax liabilities. Senate majority leader Harry Reid of Nevada has suggested that Romney paid no taxes during a 10-year period, citing an undisclosed Bain investor. He has provided no documentation on the charge, and Romney has heatedly denied it. Separately, some Democrats have questioned whether Romney has undervalued some of the assets in his IRA, but they, too, have not provided documentation.

Under current federal limits, Bain can contribute up to 25 percent of an employee’s salary, up to an annual maximum of $50,000, to employee IRAs. The Bain contributions helped partners establish IRAs that could be used as a pool of funds for the side investments, which then grow swiftly as new deals turns profits and taxes are deferred.

Bain employees, including Romney, enjoy a much richer retirement plan than most Americans. Instead of offering a 401(k) plan, in which employees set their own money aside, sometimes with a contribution from their employer, Bain funds the maximum amount it can, every year, in its employee IRAs. The accounts are managed by a Merrill Lynch adviser who regularly receives instructions from individual Bain partners about how to invest the money held in their IRAs.

The institute, at the Globe’s request, analyzed data on IRAs and calculated that only the top 0.001 percent of Americans who use these accounts have $20 million or more in them, the minimum held by Romney.

By comparison, the average size of an IRA is $67,438. Individuals with one or more IRAs have an average $91,864 in them, according to the Employee Benefit Research Institute. An estimated 48 million Americans have IRAs.

By any measure, Romney’s IRA is extraordinary. Its approximate size can be gleaned from the financial disclosure report he filed for the 2012 presidential campaign. It details the “W. Mitt Romney IRA” as holding a variety of assets worth between $20 million to $100 million. It contains 26 assets, ranging from a “Marriott Vacations” item worth up to $15,000 to a Bain-related trust worth up to $25 million.

Romney said earlier this year that “if there’s an opportunity to save taxes, we like anybody else in this country will follow that opportunity.” Michele Davis, a spokeswoman for the Romney campaign, said, “Governor and Mrs. Romney’s assets are managed on a blind basis by an independent trustee. Governor Romney’s IRA, under that trustee’s management, follows all applicable laws.” As with all IRAs, she noted, taxes are deferred, and “Governor Romney will pay taxes on amounts in the IRA when they are withdrawn.”

“Congress established IRAs for retirement savings, not as a vehicle for wealthy Americans to accumulate vast sums shielded from income tax,” said Rebecca Wilkins, senior counsel for federal tax policy at Citizens for Tax Justice, an advocacy group. She said the size of Romney’s account “raises serious questions about how it was funded — and how much income tax the Treasury is losing each year on the earnings that shouldn’t be in there.”

Romney has said he has paid taxes every year. But he has not provided documents for any year except 2010. He also provided an estimate for 2011 and said he will release those returns before the election.

The super-rich find many ways to lower or eliminate taxes, but relatively few pay none at all. The Internal Revenue Service reported that six of the 400 people with the highest adjusted gross income paid no federal income tax in 2009.

What seems clear, from Romney’s disclosure reports and interviews with his associates, is that Romney’s IRA was a centerpiece of his strategy of deferring taxes on tens of millions of dollars of Bain income.

Bain, more than other buyout firms, has made it a priority for partners to put their own money at risk alongside clients’ funds in the deals they do. As they have grown wealthier, Bain employees now account for about 10 percent of the money in their multibillion-dollar funds, far more than the industry average of 2 to 3 percent.

It’s a risk, and also an opportunity most people don’t get. By putting their money into deals, Bain employees get access to investments that have been thoroughly vetted for their robust growth prospects and get a chance to reap outsized profits.

Many Bain employees, especially in their younger years, have used their IRA funds to make these investments. And when they reaped gains on deals, the money — which must stay in the IRA until retirement to avoid penalties — would often be rolled into the next investment. With Bain generating returns of 80 percent a year in the Romney era, some partners racked up massive profits in their IRAs.

Some partners now say it is debatable if this really was a wise investment strategy. While the accounts have grown larger than many at Bain ever imagined, they will not benefit from the capital gains tax advantage Bain investors typically enjoy on investments. Instead, the money, when they withdraw it, will be taxed at ordinary income rates, currently 35 percent for the wealthiest Americans, when it is withdrawn.

Paul Beecy, a partner at the accounting firm Grant Thornton in Boston, said “there’s that trade-off” when it comes to paying the taxes now or later. But in most cases, he said, if the monies are held longer than 10 years in the IRA, the benefit of letting the money grow and paying taxes later is lucrative, often pushing the effective tax rate below 10 percent.

Overall, Romney’s 2010 returns showed that he paid a 13.9 percent tax rate, lower than many middle-income Americans. Romney’s rate was low because most of his earnings come from capital gains, which are taxed at half the high-earners’ rate of income as salary. He said last month that he would be happy to check whether he ever paid a lower rate, but his campaign has not provided further information.

Michael Kranish can be reached at kranish@globe.com. Follow him on Twitter @GlobeKranish. Beth Healy can be reached at bhealy@globe.com.

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