Brett Pfeffer, 37, a former legislative director to Rep. William Jefferson, D-La., pleaded guilty to aiding and abetting bribery of a public official and conspiracy. He could get 20 years in prison when sentenced March 31.
A spokeswoman for Jefferson declined to comment.
Specifically, Pfeffer said in federal court that a congressman demanded bribes in exchange for his assistance in brokering two African telecom deals.
Court documents did not identify the congressman by name, referring to him only as "Representative A." But the documents make clear that Jefferson is the congressman.
According to the documents, Pfeffer was employed as a legislative assistant by the congressman from 1995 through 1997. That is when Pfeffer served as a legislative aide to Jefferson, holding titles that included legislative director.
Pfeffer agreed to cooperate with prosecutors and testify if needed.
The conspiracy took place in 2004 and 2005, years after Pfeffer had left Jefferson's office, according to authorities.
Pfeffer said in court that a congressman had solicited his assistance in promoting business opportunities in Nigeria and Ghana. The congressmen demanded 5 percent to 7 percent of the newly formed African companies in exchange for his help promoting the deal to African government officials and others, according to Pfeffer.
Prosecutor Mark Lytle said the congressman and Pfeffer traveled to Ghana in July to promote a similar deal there.
Pfeffer's attorney, Paul Knight, declined to comment.
Court records give no indication how much money Jefferson stood to receive. That amount will be determined later as part of Pfeffer's sentencing.
Prosecutors have been investigating Jefferson in connection with a telecommunications deal involving a Kentucky-based company that specialized in providing high-speed Internet access over Nigeria's copper telephone wires.
The FBI raided Jefferson's home in August and, according to published reports, carted off cash from a freezer.
The FBI also raided the Maryland home of Nigerian Vice President Atiku Abubakar, seeking documents connecting him and his wife to the congressman and the telecommunications deal.
Jefferson was elected to the House in 1990, becoming the first black congressman from Louisiana since Reconstruction. He represents most of New Orleans, including sections of the city that were hardest hit by Hurricane Katrina.
He was criticized after the hurricane for receiving a National Guard escort to check on his flooded home at a time when federal resources were sorely strained.
Jefferson's spokeswoman, Melanie Roussell, declined to comment on the plea bargain except to confirm that Pfeffer worked for Jefferson in the mid-1990s. Jefferson's lawyer, Mike Fawer, said he would not comment until he had spoken to Jefferson, who was traveling in Europe.