NEW YORK — A wealthy Manhattan investment banker who was once a top fundraiser for Hillary Rodham Clinton and other big-name Democrats pleaded guilty yesterday to bank fraud.

Hassan Nemazee, 60, was arrested last year on charges that he used fake collateral to obtain more than $290 million in loans from major banks.

An indictment accused him of forging signatures, concocting bogus account statements and establishing "virtual offices" to conceal a scam in which he was using proceeds from new loans to pay off older ones — a maneuver prosecutors called a Ponzi scheme.

He entered his guilty plea at a federal courthouse in Manhattan, where he lives. Nemazee admitted to three counts of bank fraud and one count of wire fraud. The plea deal calls for him to serve at least 15 years in prison.

"I am deeply ashamed," he told the judge. "I accept full responsibility for what I've done."

Nemazee was the national finance chairman of Clinton's 2008 presidential campaign. He also raised money for President Obama and a long list of other prominent Democrats. He was Sen. John Kerry's New York finance chairman during his failed 2004 presidential run.

In 1999, President Bill Clinton nominated Nemazee to be U.S. ambassador to Argentina, but the appointment was never confirmed by the Senate, in part because of concerns about Nemazee's business dealings.

Nemazee, who had been under house arrest in Manhattan while awaiting trial, told the court that the seeds of his crime were planted when he ran into severe financial difficulties in the 1990s and tried to borrow his way out.

"As my investments went bad, the hole I dug for myself grew larger," he said.

Prosecutors suggested a more corrupt motive. Investigators said he used some proceeds of the fraud to buy property in Italy and New York and to make sizable donations to charities, political action committees and election campaigns.

The businessman had "projected the illusion of wealth ... so that he could lead a lavish lifestyle and play the part of heavyweight political fundraiser," U.S. Attorney Preet Bharara said at the time of the indictment last summer.

His sentencing was scheduled for June 30. U.S. District Judge Sidney H. Stein ordered Nemazee to report for prison by April 30, saying he could not understand why the government was not seeking detention because Nemazee had pleaded guilty and faced substantial prison time.

Assistant U.S. Attorney Daniel Levy said the government did not view Nemazee as a risk to flee, though he noted that millions of dollars were still missing.

"I have absolutely no intention of going anywhere," Nemazee told the judge.

In a release, Bharara said Nemazee "orchestrated an elaborate shell game to defraud three major banks, scamming them to the tune of $290 million to finance his posh life and create a reputation as a political kingmaker."

Nemazee's guilty plea comes six months after another prominent Clinton fundraiser, Norman Hsu, was sentenced to 24 years in prison for fraud and breaking campaign finance laws.

In that case, Hsu was accused of stealing more than $50 million from investors.

Before his arrest, he had bundled about $1 million for Democratic candidates and given another $850,000 of his own money to politicians, including Clinton.