Democratic incumbent Bill Nelson is suing Secretary of State Ken Detzner in Florida's too-close-to-call Senate race.
The lawsuit contends all mail-in and provisional ballots rejected over mismatched signatures should be counted.
The lawsuit filed in the U.S. District Court for the Northern District of Florida argues the process for validating mail-in and provisional ballot signatures is not uniform and disenfranchises voters.
Attorney Marc Elias represents the Nelson campaign.
"The problem is that voters in one county are subject to different standards for reviewing signatures than in others, and there is no uniform standard or even sufficient training for this. And it's highly error-prone."
The litigation is the latest in Florida's chaotic Senate race. Nelson's Republican challenger, Gov. Rick Scott, filed a lawsuit demanding Broward County records involving the collection and counting of ballots.
Elias also slammed Scott for enlisting the Florida Department of Law Enforcement in the race. Scott asked the FDLE to investigate elections offices in Broward and Palm Beach counties, Democratic strongholds.
"It is not appropriate for the governor of any state to suggest that he is going to use the power of the state as governor to interject his law enforcement authority to prevent the counting of ballots that have been lawfully cast."
