"That's part of the reason we have continued on with the grand jury," Assistant U.S. Attorney Paul L. Gray told U.S. District Judge Eduardo C. Robreno.

Gray's comments came during a hearing on whether Dougherty, business manager of Local 98 of the International Brotherhood of Electrical Workers, should be granted immunity so he can testify as a defense witness for electrical contractor Donald "Gus" Dougherty Jr., president of Dougherty Electric Inc.

The two are longtime friends but are not related.

Gus Dougherty is scheduled to go on trial, starting Monday, on charges that he violated the 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 officials are barred from accepting such payments, though fair-market value transactions are permitted.

Prosecutors contend that Gus Dougherty sold John Dougherty a condo in North Wildwood, N.J., for $24,000 below market value and performed $115,600 worth of electrical work at his Philadelphia home without charge.

Gus Dougherty has pleaded guilty to 98 federal charges, but not to the two counts involving John Dougherty.

Henry E. Hockeimer Jr., an attorney for John Dougherty, said yesterday that if his client is not given immunity, "he won't testify" because of the continuing investigation.

"There's no way I would recommend that he testify without protection," said Hockeimer, who called it "unfair" for John Dougherty to face "this drumbeat of an ongoing investigation where John has no ability to defend himself in any way."

Frank Keel, a spokesman for John Dougherty, later portrayed his boss as a victim of relentless federal investigators.

"The government has subpoenaed, audited and hounded this man down dead-end trails," he said. "At what point does a so-called investigation start to look like a witch-hunt?"

Eric W. Sitarchuk, an attorney for Gus Dougherty, said that John Dougherty's testimony is essential to help clear his client. He said Gus Dougherty's defense is that both transactions were fair-market value and that John Dougherty would verify that.

But Gray and Assistant U.S. Attorney Anita Eve contend that John Dougherty - whom they refer to only as "Official No. 1" - should not be granted immunity because it could jeopardize the continuing investigation of him.

"There are other issues," said Gray, "and the government's ability to prosecute those will be impinged if we're forced to have Official No. 1 testify."

Though Gray said he could not disclose more in open court, he said the inquiry went beyond the two charges at issue in the case against Gus Dougherty. "For us to be forced to be put in the situation of possibly damaging those investigations of other potential activities, which could be charged beyond the two counts . . . is unwarranted."

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

In the Iran-Contra case in the 1980s, for example, 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.

Robreno said he would rule expeditiously on the request for immunity and on other evidentiary matters.

Prosecutors have asked the judge to allow them to present evidence that Gus Dougherty had a motive to provide benefits to John Dougherty: Dougherty Electric Inc. received nearly $900,000 in subsidies between 2001 and 2005 from Local 98's job-recovery fund, which they said is administered by John Dougherty.