Defense attorneys Eric W. Sitarchuk and Nathan J. Andrisani asked U.S. District Judge Eduardo C. Robreno to give Dougherty that immunity, which would mean anything the union boss says on the witness stand could not be used against him if he is eventually charged in the case.

The defense lawyers represent Donald "Gus" Dougherty Jr., 42, president of Dougherty Electric Inc. in South Philadelphia. Gus Dougherty is a childhood friend of John Dougherty, business manager of Local 98 of the International Brotherhood of Electrical Workers. They are not related.

John Dougherty is under federal investigation for potential labor and tax violations, sources have said. He has denied any wrongdoing and has not been charged.

His attorney, Henry E. Hockeimer Jr., said yesterday that John Dougherty had not been informed by federal prosecutors that he was a target of the inquiry.

"John has not ever been informed that he's a target," Hockeimer said.

The only reason the union leader would invoke his Fifth Amendment right, Hockeimer said, was at "my recommendation in an abundance of caution" because of the "long history of a variety of investigations that John and the union have been under, which have not led to anything."

He said John Dougherty "would like to testify and not just help his friend, but to explain what happened here."

Assistant U.S. Attorneys Anita Eve and Paul Gray declined to say yesterday whether they would oppose the request for immunity.

Gus Dougherty pleaded guilty last month to 98 federal charges - but not to two other counts alleging that he made illegal payments to John Dougherty through the sale of a Shore house for $24,000 under market value, and performing $115,000 of electrical work on the labor leader's house without charge.

Gus Dougherty is slated to stand trial on those charges beginning May 19.

The charges are based on the federal Taft-Hartley Act, which prohibits the payment of money or other gifts to an officer of a labor organization that represents a contractor's employees. Union officers also are barred from accepting such payments, though fair-market transactions are permitted.

Sitarchuk and Andrisani have called it "curious" that John Dougherty has not been charged in the case - and yesterday said in court papers that the government's "choice to proceed only against Donald Dougherty, and not John Dougherty, is . . . nothing more than a matter of pure tactics.

"Perhaps it is an effort to try out the case on the lesser player first. Perhaps it is an effort to pressure Donald Dougherty," Sitarchuk and Andrisani wrote.

In any event, the defense lawyers said, the testimony of John Dougherty is essential to Gus Dougherty's defense.

"Simply put, if the jury believes him, acquittal will follow," Sitarchuk and Andrisani stated in the motion.

John Dougherty told the FBI in 2006 that he had gotten three appraisals of the property and paid above fair-market value for the North Wildwood condo, according to FBI documents submitted with the court filing.

As for the renovations to his home, he told the FBI that he had "supplied" money to Gus Dougherty, but he would not elaborate, the FBI documents stated.

In a court filing last week, prosecutors disclosed that another lawyer for John Dougherty stated in 2006 that the union leader's father-in-law provided $200,000 for some of the renovations, and sources said the money was in cash.

It is not unusual for someone under investigation to be reluctant to testify, and defense lawyers routinely advise such people - even if they are innocent - to assert their Fifth Amendment right as a precaution.

Prosecutors must be careful whenever immunity is granted to someone who gives testimony and later becomes a defendant.

In the Iran-Contra case in the 1980s, Oliver North testified before Congress and a federal appeals court later vacated his conviction, ruling that prosecutors could not show they did not make use of what he told Congress under a grant of immunity.

The U.S. Court of Appeals for the Third Circuit in Philadelphia has held that when a witness has essential exculpatory evidence, a defendant is entitled to the immunized testimony unless it is outweighed by strong government interests.

Even so, grants of immunity to defense witnesses are rare. A hearing on the issue is set for next week before Robreno.