Sen. Tom Harkin's campaign has hired a lawyer to investigate "unacceptable" conduct by a staff member allegedly involved in leaking the transcript of a strategy session held by Harkin's GOP challenger, Rep. Greg Ganske.

In a statement Thursday, attorney David Wiggins said the aide, described only as a junior staff member, was wrong to give the transcript to the media but did nothing illegal.

The aide has resigned, he said.

"I have reviewed the transcript of that meeting and it is clear that it is neither private nor confidential but simply a fund-raiser, and in that situation it certainly would not be a crime to record the meeting,'' Wiggins said.

The move came in response to allegations Thursday by Republicans that another man with ties to Harkin made a tape of the Sept. 3 Ganske meeting and gave it to the senator's campaign.

Earlier Thursday, Iowa Republican Chairman Chuck Larson claimed the alleged source was a "mole" posing as a Ganske financial supporter.

He said the campaign has turned the name of the alleged source over to investigators, who are determining whether taping the meeting was illegal. He declined to identify the man.

Two Ganske campaign officials, speaking on condition of anonymity, said the source was a Des Moines businessman who was briefly on Harkin's congressional staff in the mid-1970s. A telephone message left at the man's home was not returned.

The episode has jolted a Senate race in which Harkin was once thought to be relatively safe in his bid for a fourth term.

Harkin has apologized and said a staffer made a mistake by giving the transcript to the media.

Larson said Republicans think there are much closer links to the Harkin campaign than the senator has acknowledged.

"I think you'll find in due time that a member of the Harkin staff worked with him, provided a cassette recorder and then returned the machine after the meeting," Larson said.