Justin Lamar Sternad entered the plea Friday following his indictment for allegedly lying on campaign finance reports and conspiracy to defraud the United States, The Miami Herald reported.
Sternad -- a losing candidate in the primary to choose a Democrat to run against incumbent Republican freshman Rep. David Rivera -- came under scrutiny after the newspaper and El Nuevo Herald published stories on his campaign attacks against rival Democrat Joe Garcia, who won the primary and then unseated Rivera in November.
Sternad's campaign used a series of mailers targeted at select groups of voters, some of which hammered Garcia using attacks favored by Rivera, The Miami Herald reported Thursday.
The newspapers investigated the Sternad campaign after a finance report in August listed no campaign vendors and said Sternad raised only $11,383.60. He subsequently told the Federal Election Commission in a letter he had been "unaware of the final monetary obligation incurred by my campaign," but had since received invoices for campaign expenses.
Sternad, who is cooperating with authorities and is expected to strike a plea deal, was expected to surrender in federal court Friday and enter a not guilty plea, the report said.
The sources told the newspaper Rivera is not named in the indictment but is a target of the investigation.
Ana Alliegro, a friend of Rivera's who served as Sternad's campaign manager, delivered large amounts of cash to a campaign vendor. She was supposed to interviewed by FBI agents in September but has left the area and may have left the country, the report said.
Rivera -- who has denied doing anything wrong -- was not available for comment Thursday, the report said.
Rivera was charged in October with 11 counts of violating Florida ethics law by filing false financial disclosure forms. Prosecutors declined in April 2012 to bring criminal charges against Rivera over his campaign finances, The Miami Herald reported at the time.