By PHILIP SHENON
and DAVID STOUT
WASHINGTON, March 31 — A onetime top aide to Representative Tom DeLay, the former House Republican majority leader, pleaded guilty to conspiracy today and agreed to cooperate in a widening investigation into lobbying fraud and influence-peddling.
Tony C. Rudy, Mr. DeLay's former deputy chief of staff, entered the plea in United States District Court here. The felony charge carries a maximum sentence of five years in prison. Prosecutors asked that sentencing be deferred until Mr. Rudy is finished telling them what he knows.
"Guilty, your honor," Mr. Rudy, 39, replied when Judge Ellen S. Huvelle asked him how he pleaded. Mr. Rudy appeared calm and replied to questions with simple "yes" and "no" answers.
Mr. Rudy was the third major figure in the affair to plead guilty, and the first since Jack Abramoff, the powerful and wealthy Republican lobbyist, pleaded guilty to three felonies in January in exchange for a reduced sentence of about 10 years, a promise to pay tax penalties and make restitution to former clients he defrauded and cooperate with the investigation.
Michael Scanlon, a former aide to Mr. DeLay and a former partner of Mr. Abramoff, has also pleaded guilty and is cooperating.
The affair in which Mr. Rudy was caught up is widely believed to be focusing on a dozen or more members of Congress, some of whom were wined and dined by Mr. Abramoff and accepted lavish trips arranged by him. The growing scandal has sparked calls for restrictions on spending by lobbyists and acceptance of favors by lawmakers.
The plea agreement Mr. Rudy entered today does not contain any allegations against Mr. DeLay, who has denied any wrongdoing.
As part of the arrangement, prosecutors agreed not to pursue possible charges against Mr. Rudy's wife, Lisa. When Mr. Abramoff pleaded guilty in January, documents described a payment of $50,000 from Mr. Abramoff to Ms. Rudy through a charity organization in exchange for her husband's "agreement to perform a series of official acts."
Mr. Rudy was mentioned in the paperwork accompanying Mr. Abramoff's plea agreement as "Staffer A." Investigators were known to be looking into whether Mr. Rudy helped secure legislative favors for Mr. Abramoff's clients in exchange for gifts and the promise of a future job while he was still on the congressman's staff.
Mr. DeLay has repeatedly denied any wrongdoing, not only in matters related to Mr. Abramoff but in an unrelated campaign-finance case in Texas, which he has derided as the work of a publicity-happy and partisan prosecutor.
Shortly after Mr. Abramoff pleaded guilty in early January, one of Mr. DeLay's lawyers, Richard Cullen, said that if it became clear that Mr. Rudy had done anything improper, "Tom DeLay is going to be very sad and very disappointed. Because he will feel betrayed, and he expects much, more more from his staff than activities like that."