US lobbyist Jack Abramoff, 46, apologised for his actions during a brief appearance in the US District Court, where he expressed "profound regret for the multitude of mistakes and harm I have caused.
"I only hope that I can merit forgiveness from the Almighty and those I have wronged," said the former powerbroker in the Washington lobbying community.
He and an associate are said to have defrauded several indigenous tribes of millions of dollars.
Abramoff is expected to now cooperate in an investigation which threatens a number of powerful members of the US Congress, including Tom DeLay, the former Republican majority leader of the House of Representatives.
30 years jail
Abramoff could face up to 30 years in prison and restitution of US$25 million, but is expected to receive a significantly lesser sentence in exchange for his testimony against former associates in Congress and their aides.
Judge Ellen Segal Huvelle was asked by prosecutors to defer sentencing until Abramoff has finished cooperating with the authorities.
The next hearing has been set for March 24.
Court papers filed on Tuesday alleged that Abramoff and his former business partner Michael Scanlon conspired to "corruptly give, offer and promise things of value, including money, meals, trips and entertainment" to public officials.
The purpose of the gifts was "the intent to influence, and in return for agreements to perform, official acts."
The indictment against the pair accuses them of putting millions of dollars, wrongfully claimed from American Indian tribes, into a fund for bribing lawmakers.
Gifts and favours were funnelled to a member of Congress identified only as "Representative #1" and his staff, included a trip to Scotland's world famous golf courses, tickets to sporting events and meals at Abramoff's swank Washington restaurant, according to the indictment.
Published media reports have identified Republican congressman Robert Ney of Ohio as having received favors including golf trips and meals, however Mr Ney has accused Abramoff of misleading him.
Mr DeLay, who stepped down as majority leader after being indicted on money laundering charges in his home state of Texas, has also been linked with Abramoff.
Election worries
Abramoff's case has been watched closely by both sides of politics in Washington.
US President George W Bush's Republican Party is particularly concerned that the case, with its taint of corruption, might hinder its chances in mid-term elections in November.
The party has been hit by a string of other headaches, including an insider trading probe against Senate Majority leader Bill Frist.
The White House was rocked last year by the indictment of Vice President Dick Cheney's top aide I Lewis 'Scooter' Libby on charges arising from a probe into the outing of CIA spy Valerie Plame.
Mr Bush's spokesman Scott McClellan on Tuesday took care to distance the White House from Abramoff, as reports surfaced of the plea deal.
Abramoff is expected to plead guilty on Wednesday to charges in another probe, in Florida, in which he faces charges of conspiracy and wire fraud in connection with a casino deal and faces up to 10 years in jail if convicted.