Mike Slanker, who served as the chief political adviser to Ensign when Ensign ran the National Republican Senatorial Committee, was subpoenaed by the Senate Ethics Committee, which is investigating the case.
Ensign has admitted to an extramarital affair with the wife of one of his former aides, Douglas Hampton, who has gone on national television to talk about the controversy.
Ed Patru, a spokesman for McMahon, confirmed on Saturday afternoon that Slanker had received a subpoena, and also that Slanker has been a general consultant for the McMahon campaign since it started in mid-September. Slanker is based at his firm in Nevada, and does not work in McMahon's Connecticut headquarters.
"Ultimately, this campaign is not about staff or consultants or anybody but Linda and her vision for changing the direction of Washington and putting people back to work in Connecticut," Patru said.
He said it was not unusual for investigators to question or subpoena records of people who had been close to a political figure.
"The standard procedure for the ethics committee is to cast a pretty broad net," he said.
McMahon is expected to spend as much as $50 million of her own money in the race to unseat incumbent Democrat Christopher J. Dodd, a U.S. senator for nearly 30 years.
She is seeking support from delegates to the Republican convention, which will be held in Hartford in May, against former U.S. Rep. Rob Simmons of Stonington and Fairfield County investor Peter Schiff. A primary, if necessary would be held in August 2010, before the general election in November.