Romney Riches Are Being Seen as New Hurdle With a fortune estimated to be as large as a quarter of a billion dollars, Mitt Romney is among the wealthiest men ever to run for president. Mitt Romney at Hudson's Smokehouse in Lexington, S.C. But attacks from Democrats and Republicans over his career in the leveraged buyout business and his reluctance to release his tax returns have underscored another central fact about Mr. Romney: The wealth that has helped underwrite his career in politics remains shrouded in considerable secrecy, which now poses a major political risk on the campaign trail. Mr. Romney's finances are complex and far-flung. He and his wife, Ann, have reported holdings in dozens of publicly traded companies, mutual funds and high-end investment partnerships, with much of their family wealth held in blind trusts that conceal their full size from public view. And Mr. Romney's disclosure on Tuesday that he pays an effective tax rate of about 15 percent on his income focused new attention on some aspects of his finances, including his millions of dollars in donations to the Mormon Church and his continuing compensation from Bain Capital, the private equity firm he left more than a decade ago. Just three days ahead of the South Carolina primary, Mr. Romney's opponents on Wednesday vigorously exploited uncertainty about his finances by demanding that he release his tax returns before their party picks a nominee, while Democrats, previewing likely attacks in a general election campaign, accused Mr. Romney of having something to hide. The matter is also leading to more scrutiny of his tax proposals. "We can't fire our nominee in September," Gov. Rick Perry of Texas said on Wednesday on "Fox and Friends." Whether the attacks will help or hurt Mr. Romney with voters remains to be seen. And Romney aides said privately that they were prepared for the attacks on Mr. Romney's wealth, which they believe will backfire, aligning Mr. Romney's critics with liberals and rallying more Republicans to his side. But even some allies of Mr. Romney's raised concerns: Gov. Chris Christie of New Jersey, one of his top supporters, said in interviews on Wednesday morning that Mr. Romney should make the returns public "sooner rather than later." If there is one thing Mr. Romney has been consistent on over the years, however, it is taxes. In his campaigns for the United States Senate, governor of Massachusetts and president, he has never once released his tax returns - not even in 1994, when he made a major issue out of Senator Edward M. Kennedy's refusal to release his own returns. Mr. Romney has not pledged to release past returns, and the one he has suggested he would release, in April, is a work in progress. At the end of last year, he could have arranged to have some of his compensation deferred. There are a variety of mechanisms, like grantor retained annuity trusts, that athletes, entertainers and businesspeople routinely use to push their income and tax liability into the future and spread it out over years. An examination of Mr. Romney's public financial disclosures and other documents reveals some outlines of his wealth and potential tax liabilities. Even if nearly all of his income was taxed at the 15 percent rate levied on dividends and long-term capital gains, Mr. Romney would be eligible to lower his tax bill by deducting charitable donations, property taxes, and state and local taxes. Mr. Romney practices tithing, donating at least 10 percent of his income to the church, and he is entitled to deduct those contributions from his income. But Mr. Romney, who has had to contend with a bias that some voters harbor against Mormons, has said little about his millions of dollars in donations to the church. Public records show that Mr. and Mrs. Romney have given $9.5 million to their family foundation since 1999. The foundation, in turn, has donated $4.7 million to the Church of Jesus Christ of Latter-day Saints, and $300,000 to Brigham Young University, which is Mr. Romney's alma mater and is owned and operated by the church. The church contributions came in several large chunks, including $1.8 million in 2008 and $1.9 million in 2003. In addition, records from the Securities and Exchange Commission show that the Mormon Church has reaped more than $13 million over the last 15 years by selling shares in companies that Bain Capital invested in, including Burger King Holdings and Domino's. Reporting was contributed by Stephanie Strom and Laurie Goodstein from New York, and Michael D. Shear and Kitty Bennett from Washington.