Their statement was filed in court papers two days after lawyers for a longtime friend said they needed Dougherty's testimony to help win the case against their client, South Philadelphia electrical contractor Donald "Gus" Dougherty Jr.

The lawyers, Eric W. Sitarchuk and Nathan J. Andrisani, said John Dougherty would assert his Fifth Amendment right against self-incrimination unless given immunity to testify as a defense witness.

If he is granted immunity and eventually charged, nothing he says on the stand can be used against him.

John Dougherty is business manager of Local 98 of the International Brotherhood of Electrical Workers. Last month, he lost his bid for the Democratic nomination to replace State Sen. Vincent J. Fumo in Philadelphia's First District.

Henry E. Hockeimer Jr., an attorney for John Dougherty, questioned yesterday how much longer the investigation would continue and said it was unfair to his client.

"These are issues they have been looking at now for over two years," Hockeimer said. "They continue to hold this cloud over John's head, and the reasonable question at some point is, 'When is enough enough?' "

Gus Dougherty, 42, president of Dougherty Electric Inc., is accused of violating the federal Taft-Hartley Act by selling John Dougherty a North Wildwood condo for $24,000 under market value and by performing $115,600 worth of electrical work without charge at his Philadelphia home.

He pleaded guilty last month to 98 other federal charges - but not to those two counts involving John Dougherty. Trial on those charges is set to begin May 19 before U.S. District Judge Eduardo C. Robreno.

The prosecutors, Assistant U.S. Attorneys Anita Eve and Paul L. Gray, said in the court document that John Dougherty - whom they referred to as "Official No. 1" - was a subject of the federal investigation.

The government, they stated, "has a strong and legitimate interest in the continued investigation and possible future prosecution of Official No. 1 for the same offenses with which Donald Dougherty is charged, and others, with whom counsel for Donald Dougherty is unfamiliar."

That investigation, they stated, "could be jeopardized by a premature grant of use immunity."

Said Hockeimer: "For whatever reason, they managed to keep John as the elephant in the room here. It's very unfair."

Frank Keel, a spokesman for John Dougherty, said he also wanted to comment on the prosecution filing.

"This seems like an attempt to destroy a man through a million little nicks and cuts. For a decade now, there have been numerous allegations against John that have gone nowhere," he wrote in an e-mail late yesterday.

The prosecutors also cited what another lawyer for John Dougherty stated in a 2006 letter: that his father-in-law had paid $200,000 for some of the renovations, with John Dougherty adding $52,000.

At trial, the prosecutors stated, they will introduce the letter, "which asserted that Official No. 1 had paid for the renovations (and presented an explanation which the government will prove is false)."

For prosecutors, immunity can be a tricky proposition for someone who testifies and is later charged because they must show that their underlying evidence is not based directly or indirectly on that testimony.

Lawyers for people under investigation routinely advise their clients - even those who have done nothing wrong - against testifying without immunity.