Sternad, who ran against Rivera's Democratic rival, Joe GarciaJose (Joe) Antonio GarciaHispanic Caucus to invite Republicans to join Vulnerable House incumbents build up war chests Florida Dems hosting fundraiser for GOP lawmaker MORE, in the primary and lost, told authorities that his currently missing campaign manager, Ana Sol Alliegro, acted as a liaison between his campaign and Rivera. He said she referred to the congressman by his initials or called him by the nickname "The Gangster."

He said that he has not met Rivera, as Rivera alleged, but that he realized the congressman was the source of monetary support after examining Alliegro's Facebook and Twitter accounts and seeing photos of the two together.

“We will respond when these so-called ‘sources’ are willing to go on the record,” attorney Michael Band, who represents Rivera, told the Herald. “We are not going to respond to unfounded rumors and innuendo. My client is in the middle of an election and it’s unfair for us to be shadow-boxing with unnamed sources.”

Sternad is the subject of a federal investigation into his campaign finances, which far exceeded what he would have likely been able to personally contribute on his $30,000 annual salary. Though he reported taking in less than $11,400, his campaign issued a dozen highly targeted campaign mailings that would have cost thousands.

Sternad, with Alliegro's help, amended campaign finance reports to reflect that he loaned himself $53,000 more than he originally reported.

Sternad explained that he went along with the scheme because he was told that, even if he lost, the "D.R." figure behind the campaign would find him a better job. He says he is cooperating with authorities to try to receive more lenient treatment from the judge in the case.

Garcia went on to win the Democratic primary to meet Rivera for a rematch of 2010 this November. Though he lost to the congressman by nearly 10 points that year, he's running a stronger campaign this year with support from the national party, and the developing investigation surrounding Sternad could be hurting Rivera's standing in the district.

—This story was updated at 9:47 a.m.