Democrat Bill Nelson is suing the Florida Secretary of State and demanding the state count all provisional and mail-in ballots deemed to have a signature mismatch.
The legal challenge confronts a Florida law that requires county election officials to double check signatures of voters who cast provisional and mail-in ballots, and reject any that don't match state records.
Nelson's recount lawyer Marc Elias said the law puts the responsibility of signature review in the hands of untrained, unqualified local election workers and leads to the disqualification of legitimate ballots.
"This results in a complete lack of uniformity," Elias said. "Voters in one county are subject to one standard for reviewing signatures than others."
Nelson is asking the U.S. District Court for the Northern District of Florida for an immediate injunction and to declare "that all voters who submit a (vote-by-mail) or provisional ballot, and whose ballots are subsequently determined to involve a signature mismatch, be counted as valid votes." He also requests Saturday's deadline to canvas ballots be extended until the legal matter is resolved.
Similar action has been taken in other states with signature match laws, most notably Georgia.
The lawsuit, filed Thursday, lists the Democratic Executive Committee as a plaintiff with Nelson and Florida Secretary Ken Detzner as a defendant.
The legal action comes as Nelson is headed toward a recount in his Senate race with Republican Gov. Rick Scott. As of Friday morning, Scott led Nelson by 15,079 votes, a margin that continues to narrow since election night.
Nelson's camp believes provisional and mail-in ballots will favor their candidate and tighten the contest even more before a recount begins on Sunday.
Scott, who yesterday sued two South Florida election supervisors, vowed to meet Nelson's challenge in court.
"With today's filing, their desperation has driven them to ask the federal courts to allow voter fraud," Scott campaign manager Jackie Schutz Zeckman said.
This story is developing.