Wisconsin Democrats say Johnson failed to disclose the specific terms of that agreement on the financial disclosure reports that he and other members of Congress must annually file. The complaint also questions whether Johnson actually had such an agreement and says if he did not, the $10 million payout would have been an illegal contribution to his campaign.

Johnson’s campaign said he properly disclosed the payout on his 2010 financial disclosure report and called the complaint an attempt to distract from Republican allegations against Feingold.

The campaign said the payout came after Johnson took no salary as PACUR CEO for 13 years “to make sure PACUR had the resources to grow.”

“Ron Johnson followed the law and disclosed his back pay five years ago,” Johnson spokesman William Allison told the Wisconsin State Journal.

Critics have dogged Johnson about the $10 million payment since it became public in 2011. The Democratic complaint filing is the latest in a recent tit-for-tat of allegations between allies of Johnson and Feingold — not an uncommon series of developments in the late stages of a campaign.