A once-wealthy businessman and prominent donor to Democratic candidates pleaded guilty on Thursday to stealing hundreds of millions of dollars to buy property in Westchester County, donate to charity and give money to political campaigns.

“I am deeply ashamed of my conduct,” the man, Hassan Nemazee, 60, told Judge Sidney H. Stein in Federal District Court in Manhattan. “I accept full responsibility for my actions.”

In September, federal prosecutors charged Mr. Nemazee, who had raised money for Bill and Hillary Rodham Clinton, John Kerry and Barack Obama, with running a Ponzi scheme from 1998 to 2009 that obtained $292 million in fraudulent loans from Bank of America, Citibank and HSBC.

At the time of his arrest on Aug. 25, Mr. Nemazee, the Iranian-American son of a shipping magnate, was chairman and chief executive of Nemazee Capital, a holding company with investments in private and public companies. In pleading guilty to three counts of bank fraud and a count of wire fraud, Mr. Nemazee, of Manhattan, told the judge that his misdeeds began when he was facing “severe financial difficulties.”