Renacci spokesman Shawn Ryan said the U.S. Attorney's Office contacted the campaign four or five months ago with questions about donations from employees of Benjamin Suarez, a direct-marketing magnate, to either the congressman's campaign or Mandel's.

Renacci, seeking a second term representing Ohio's 16th Congressional District, is being challenged by Rep. Betty Sutton, a Democrat who was drawn out of her district. Mandel, elected treasurer in 2010, is challenging Democratic Sen. Sherrod Brown.

The (Toledo) Blade had reported last August that Suarez and 16 employees and spouses from Canton-based Suarez Corporation Industries gave donations totaling $100,000 each to Renacci and Mandel.

Federal campaign finance law prohibits a donor from contributing in someone else's name. Corporations also are prohibited from awarding bonuses or other rewards to employees in exchange for campaign contributions. The company has said the employees gave freely and weren't compensated.

Ryan said the government sought records showing the dates of donations from Suarez employees.

"The attorney had apparently thought the donations had all been made together at the same time, which they hadn't," Ryan said in an email. "They came in over the course of a few months."

Renacci's campaign treasurer provided all the information to the attorney and that was the last time they spoke to him, Ryan said. He said any contributions would be returned immediately if they are determined to be improper.

"To our knowledge, no contributions made by any donor to our campaign were made improperly," Ryan said in the email. "However, if we find out that any contributions, either made in the past or in the future, were not made properly and in full accordance with the law, they will be returned immediately."

Mandel campaign spokesman Travis Considine also confirmed that donor information was sought.

"The campaign is aware of the investigation and is fully cooperating," he said. "Neither the campaign nor anyone associated with it is a target of the investigation."

Considine said the campaign was setting aside in a separate account the roughly $100,000 in donations from Suarez employees pending the outcome of the investigation. He the money may be returned or donated to charity.

Mike Tobin, a spokesman for the U.S. Attorney's Office in Cleveland said he could not confirm or deny an investigation.

A message left Monday with Suarez's attorney was not immediately returned.

The New Republic first reported on the existence of a federal investigation into the campaigns.