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.

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.