Ross argued that Johnson has signed a retainer agreement with the Wisconsin Institute for Law and Liberty and attorney Rick Esenberg, and as a result his campaign is required to report at least an estimated amount, which it could amend when the actual amount is known.

One Wisconsin Now pointed Wednesday to FEC guidance for congressional candidates and committees on federal campaign finance law, which says a "written agreement to make an expenditure, such as a media contract, constitutes an expenditure."

And the group pointed to other FEC guidance that says if "the exact amount of a debt is not known, the committee reports an estimated amount," which it can amend once the correct amount is known.

Ross warned that Johnson’s could hide such information from the public for years under his interpretation of disclosure requirements.

"Sen. Johnson is getting a sweetheart deal from a right-wing funded legal outfit to sue to take away people’s health care and eliminate protections from insurance company abuses, and he’s trying to fatten his campaign account while he’s at it," Ross said. "He at least owes the public the proper disclosure, not excuses."