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.

Cochran said he spoke numerous times with Rivera, produced the lists of targeted voters and emailed it to Rapid Mail and Computer Services in Hialeah, which mailed the fliers. Cochran cc’d the owner and Rivera in a July 29 email, which he provided to The Herald.

When contacted by the Herald for comment, the Rivera campaign responded via email Tuesday night: “Congressman Rivera has never met or spoken to Mr. Sternad and knows absolutely nothing about him or his campaign.”

But Rivera’s campaign acknowledged he might have received an email from Campaign Data intended for Sternad’s use.

“Anything Campaign Data mistakenly sent to Congressman Rivera was done so in error, which has occurred previously, and without Congressman Rivera’s knowledge or consent,” Rivera’s campaign said.

Sternard and his attorney declined comment.

John Borrero, president of Rapid Mail, declined comment on Monday and Tuesday.

But late last week he told The Herald that Rivera was directly involved in the Sternad campaign mailings — a fact backed up by numerous sources with knowledge of the operation.

Interviews with campaign sources, invoices, campaign records and other documents show that Rivera personally and frequently called Rapid Mail about Sternad’s mailers. During one call, Rivera directed an employee to walk outside, check the office mailbox for an envelope containing payment for one mailer., the sources said.

The envelope was stuffed with cash — $7,800.

Last week, Borrero told The Herald that the Sternad campaign had paid cash for six of the cash mailers, which cost between $4,000 and $6,000 each. He said he was surprised by the amount of cash, which he sometimes may see from private clients and not usually campaigns.

“I never saw so much cash,” Borrero said last week.

Since last week, however, The Herald has learned that Sternad sent more mailers funded by more secretive cash. The campaign mailed at least 11 fliers totaling at least $43,000.

Nearly all of the mailers were paid for in cash as well. One was paid with a check from a third party vendor, sources told the Herald.

Experienced campaign workers and campaign-law experts say they’ve never heard of a campaign paying cash for mailers.

“I’ve never heard of anything like this,” said Mark Herron, a veteran elections-law lawyer who has represented more than scores of politicians and candidates.