Howard J. Cain, 59, began cooperating after he came under scrutiny for possible tax violations. If he testifies, his defection would be the first from Fumo's historically tight inner circle.

Fumo, 64, a powerful Democrat in South Philadelphia and Harrisburg, is scheduled to stand trial in September on a sweeping federal corruption indictment. He has pleaded not guilty and insists he has done nothing wrong.

The pressure on Cain was a sign that prosecutors, the FBI and the IRS have continued to seek to strengthen their case against Fumo during the long wait for the start of his trial.

Fumo sought and won a year's delay in the trial after he dropped his longtime ally Richard A. Sprague as his lawyer and hired veteran Philadelphia lawyer Dennis J. Cogan.

Cogan and Cain's attorney, Peter J. Scuderi, declined to comment last night. Cain did not return a phone call.

FBI spokeswoman Jerri Williams declined to comment. Assistant U.S. Attorneys John Pease and Robert A. Zauzmer, who are to prosecute the case, also would not comment. "We never confirm or deny anything regarding a pending matter or the absence of a pending matter," Zauzmer said.

While the prospect of a Fumo insider's turning against his old comrade appeared to be a coup for prosecutors, it was unclear what Cain has been telling authorities. Nor could the nature of his possible tax issues be learned.

Shortly after his March release from a hospital after a heart attack, Fumo announced that he would retire from the Senate this fall after 30 years in office.

In an interview shown April 20 on the Pennsylvania Cable Network, Fumo said he was confident of victory in court.

"We're ready to roll," he said. "We're ready to walk away with a 'not guilty.' "

Fumo, a Democrat from South Philadelphia who has long been a power in city and state politics, derided the indictment in the interview:

"It's filled with half-truths and things that no one ever would have considered would be a crime - and that we still don't consider to be a crime."

Fumo has long been close to Cain, an amiable man with a booming voice who is widely respected for his political savvy. In court filings, prosecutors cite e-mails between the pair on subjects ranging from polls to postelection plans.

"They are very close," a Fumo ally said. "I would put him down as one of Vince's most trusted people."

The 139-count indictment against Fumo alleges that he defrauded the state Senate and two nonprofits, and attempted to thwart the federal investigation with a coverup.

One specific allegation is that he illegally paid Cain with taxpayer money for political work. The government says the public money should have been used only to pay for legislative tasks.

In all, the indictment says, the state Senate paid Cain $463,500 between 2000 and 2005.

"During these years," the indictment says, Cain "assisted Fumo extensively and continuously in the campaigns of Fumo and many others whom Fumo supported."

In another part of the indictment, the grand jury alleges that Cain was part of a scheme to disguise the funding of a lawsuit against a top Republican state senator who long had been a Fumo nemesis.

In 2001, the government says, a nonprofit controlled by Fumo paid $20,000 to Cain's consulting business. Cain kept $3,000 and paid the rest to a lawyer handling the lawsuit.

Cain is not charged in the indictment.

In a biographical sketch, Cain said he had worked on than 80 political campaign since the late 1970s. He said his skills included get-out-the-vote drives, advertising and media, and computerized election analysis.

Aside from Fumo, Cain over the years has advised U.S. Rep. Robert A. Brady (D., Pa.), former City Controller Jonathan Saidel, onetime Philadelphia mayoral candidate John F. White Jr., and - in rare support for a Republican - mayoral candidate Sam Katz.

Cain recently told an acquaintance, however, that he had gotten out of the political game in the last couple of years.

Moreover, the Philadelphia Parking Authority just terminated his $3,000-a-month contract as a consultant. Since 2001, the authority had paid Cain about $200,000 in that role.

In past years, Cain also drew a $60,000 yearly paycheck as president of a nonprofit known as the Ship Recycling Research Institute. It has been funded by another organization tied to Fumo.

Cain has said the institute was founded in 2002 to promote "ship recycling" - scrapping and reusing obsolete maritime vessels, sometimes to build artificial reefs.

In a 2004 interview, Cain said the institute had yet to complete a deal.

The institute got a $500,000 loan from the Delaware Valley Regional Economic Development Fund. The fund gives out grants and loans, using money that Fumo arranged for it to obtain from Peco Energy.

In 1998, during negotiations with Fumo over the deregulation of power sales statewide, Peco agreed to a system under which a few cents from electricity bills would go to nonprofits for economic development. One was the Delaware Valley Regional Economic Development Fund.

In January 2004, The Inquirer reported that the fund's board was made up of people who were political or business allies of Fumo, and that most of its grants and loans had gone to people with connections to Fumo or the fund's board.

In 2004, federal authorities subpoenaed records from the fund, shortly after its investigation of Fumo began.