Fueled with $43,000 in secret money, Republican Rep. David Rivera helped run a shadow campaign that might have broken federal laws in last week’s Democratic primary against his political nemesis Joe Garcia, according to campaign sources and finance records. As part of the effort, a political unknown named Justin Lamar Sternad campaigned against Garcia by running a sophisticated mail campaign that Rivera helped orchestrate and fund, campaign vendors said. Among the revelations: The mailers were often paid in envelopes stuffed with crisp hundred-dollar bills. Rivera and Sternad have denied working together in his campaign, which ended Aug. 14. But Hugh Cochran, president of Campaign Data, told The Herald this week that Rivera contacted him in July and requested he create a list of voters who were ultimately targeted in the 11 mailers sent by Sternad’s campaign. “David hired me to run the data,” said Cochran, who is a retired FBI agent.