WASHINGTON (AP) — Republican presidential candidate Mitt Romney has criticized rival Newt Gingrich for earning more than $1.6 million in consulting fees from Freddie Mac even though he has as much as $500,000 invested in the U.S.-backed lender and its sister entity, Fannie Mae.A day before Romney planned to release his income tax returns, his old investments in two controversial government-backed housing lenders stirred up new questions at the same time his campaign targeted Gingrich for his work for Freddie Mac.The dimensions and the sources of Romney's wealth, which he has estimated to be as much as $250 million, have become pivotal issues in the roiling GOP primary campaign. For months, Romney dismissed calls to release his personal income tax records. But after mounting criticism from his rivals and others, coupled with his stinging weekend loss to Gingrich in the South Carolina primary, Romney agreed to release his 2010 return and 2011 estimate.Romney's most recent financial disclosure report listed several investments in U.S.-backed lenders Fannie Mae and Freddie Mac. Romney, Gingrich and other GOP critics repeatedly have all singled out the two quasi-government entities as prime villains in the housing crisis that played a central role in the nation's long and deep recession.While continuing to hammer Gingrich for his consulting work for Freddie Mac, the Romney campaign sought to deflect questions about the former Massachusetts governor's investments. They include a mutual fund worth up to $500,000 that includes assets from both lenders among other government income, and separate investments in each of the lenders in Romney's individual retirement account, each worth between $100,000 and $250,000.Romney campaign officials said Monday that a trustee handles the investments and that Romney had no role in choosing or managing them.The tax returns Romney planned to release Tuesday could provide new details about his investments and his annual take as founder of the Bain Capital private equity firm. Gingrich released his own 2010 federal tax return last weekend, during a South Carolina GOP debate, and his campaign said he would disclose his full contracts with Freddie Mac on Monday night just before the debate in Tampa, Fla.Romney's tax returns are likely to sketch out critical information about the tax strategies he employs. Tax experts said these likely include his use of a low 15 percent capital gains rate to reduce the taxes he pays on dozens of large investments that flow into his blind trust, charitable donation strategies that benefit philanthropies but also further reduce his tax burden and investments routed through offshore affiliates that could help him defer some tax payments.Romney already has acknowledged that his current tax rate is about 15 percent, a level far lower than standard rates for high-income earners and similar to the capital gains rate. But some tax law and tax policy experts suggest that Romney likely has paid similarly low rates throughout his Bain years, continuing through the 13 years since he left the firm.Joseph Bankman, a Stanford University business and law professor who has testified before Congress on the taxes paid by private equity firms like Bain, said Romney's background as a financier, coupled with his growing wealth and ability to use sophisticated tax tactics, makes it highly likely that he has paid taxes at the capital gains rate for most of his career."There is no reason to believe that Romney ever paid more that the going rate for capital gains," Bankman said.The current lowest rate for long-term capital gains is 15 percent, but a higher rate of 20 percent had been in effect since 1981 until President George W. Bush signed into law a massive tax cut program in 2001.Romney's 2010 return and 2011 estimate, Bankman said, could detail whether he continues to make any "carried interest," a lucrative investment arrangement typical among private equity managers that earns at least 20 percent of an investment fund's profits. The bulk of Romney's profits from his "carry," as the maneuver is often called in the private equity world, came during his tenure as Bain's founder and managing director in the 1980s and 1990s, but reportedly continued in the years after he left the firm.At least six of Romney's investments, worth between $5 million and $25 million, were made in funds that have offshore affiliates based in the Cayman Islands, a well-known haven for companies seeking to attract foreign and non-profit investors. One of those funds, which is invested in Romney's retirement IRA, could be used to defer some of his tax payments, Columbia University law professor Michael Graetz said. It is uncertain if any offshore accounts would be identified in Romney's new tax disclosures.Romney's vast investments contain other funds than the ones he profited from as a Bain Capital executive. But it was unclear Monday whether he had any direct role in handling the investments in Fannie Mae and Freddie Mac that appear on his 2012 presidential disclosure.One investment, listed as a "Federated Government Obligation Fund" and worth between $250,000 and $500,000, was a mutual fund that included both Fannie Mae and Freddie Mac assets among a larger pool that included other government securities.The holding was not listed in Romney's blind trust, which led some Democratic Party activists to suggest that the investment was under his direct control."He is relentlessly attacking Newt Gingrich over his ties to Freddie Mac despite the fact that he personally invested up to a half a million dollars in both Fannie Mae and Freddie Mac," said Ty Matsdorf, a senior adviser with American Bridge 21st Century, a PAC associated with Democratic Party and liberal causes.Former GOP Rep. J.C. Watts, a Gingrich supporter, said Monday that Romney was on a slippery slope calling his opponent a lobbyist and raising doubts about Gingrich's work for Freddie Mac. But he did not directly address Romney's investments with the lender or with Fannie Mae."Some might see it as splitting hairs. But Newt Gingrich was not walking the halls of House and Senate," Watts said on a conference call arranged by the campaign. "He was never doing the hand-to-hand combat doing the lobbying, consulting, whatever you want to call it."A Romney campaign official who insisted on anonymity to discuss that investment in greater detail said that Romney's trustee had bought the government investment fund in 2007, before the housing crisis broke.The Romney official said that the government fund was purchased through a charity trust that does not appear in Romney's presidential disclosure but will show up on his income tax return for 2010. That trust, called a Charitable Remainder Unitrust, is a standard tax strategy among the wealthy that provides investors with a fixed payout each year. What remains in the account at a later date, or when the investor dies, is turned over to charity, the official said.Romney does not directly control the investment account, Romney campaign senior adviser Eric Fehrnstrom said earlier on Monday. "His investments are controlled by a trustee," Fehrnstrom said.Separately, Romney's IRA retirement account lists both a Fannie Mae and a Freddie Mac security, each worth between $100,000 and $250,000. But because those are in Romney's IRA, they also appear to be under control of the trustee.Tax experts said Romney's income tax returns may contain other charity structures and tax strategies designed to both boost his income and charity donations, while minimizing his involvement because of his presidential ambitions.Roberton Williams, a senior fellow at the Tax Policy Center, a branch of the nonpartisan Urban Institute in Washington, said much can be gleaned by looking at Romney's sources of income and his itemized deductions. The latter would include Romney's 10 percent annual tithing to the Mormon Church, which would lower his tax liability and counteract higher taxes he would otherwise pay on non-investment income, like speaking fees.An annual study of charity giving by the ultra-rich has shown that tax strategies are only one of several motivations, said Una Osili, a professor of economics and philanthropic studies at Indiana University. The most recent 2010 study of "high net-worth philanthropy" found that religious ties and volunteer and donor relationships are also important, said Osili, director of research for the studies.Osili noted that more than 90 percent high net-worth donors tend to make donations in either cash or checks. But Romney's own family charitable foundation, the Tyler Charitable Fund, has showed signs that Romney has also donated stock investments to charity — and his 2010 returns could provide more evidence of that trend.___Associated Press writers Kasie Hunt and Brian Bakst in Tampa contributed to this report.