An already nasty Miami-Dade County Commission campaign appears poised to get worse.

Candidate Juan C. Zapata is bracing for the latest attack, which he predicts will bring up a federal money-laundering conviction against his parents 30 years ago and his own brush with the law as a college student 20 years ago.

Political advertisements bringing up that history have yet to hit mailboxes or the airwaves. But Zapata, a former state representative, told The Miami Herald that he suspects a shadowy third-party group in Tallahassee that has already bankrolled several mailers against him is preparing to send out new ones as absentee ballots begin to arrive in voters’ homes.

“The strategy is to destroy Zapata,” he said. “ ‘Let’s point out the two or three blemishes on his record and go after him.’ ”

His opponent, Manny Machado, a Miami-Dade police detective, said Zapata was “victimizing himself” by mentioning potential attacks.

“I don’t know where they’re from, but obviously he may not have people who want to see him in office,” Machado said. “I have no reason to go negative.”

Machado decried two mailers against him that called him a “bureaucrat” and that suggested county unions are bankrolling his campaign. The fliers were sent by Citizens for Progress & Integrity, a local electioneering communications organization (ECO) that backs Zapata.

The two will face off in a nonpartisan Nov. 6 runoff to replace outgoing Commissioner Joe Martinez in District 11, which stretches across a wide swath of unincorporated West Miami-Dade.

The most recent mudslinging involves Voters Response, a state ECO that has mailed at least four fliers attacking Zapata as a tax-raiser and phony Republican.

Zapata says Voters Response, headed by Tallahassee lawyer and lobbyist David Ramba, is doing the bidding of the Police Benevolent Association and of state Rep. Frank Artiles, a fellow Miami Republican who twice lost legislative elections to Zapata. One of the fliers mentions that Zapata endorsed Artiles’ Democratic opponent in 2010.

Artiles, who is unopposed in November, scoffed at the suggestion but did not deny any involvement. He called Zapata out of touch and repeated the accusations contained in the mailers.

“[I]t’s not surprising that Mr. Zapata would try and divert the conversation away from the real issues in District 11,” Artiles wrote in an email.

Zapata countered that it is the attacks that keep the race from focusing on substantive matters.

Voters Response has recently been funded by several other political committees that have contributed to a slew of lawmakers, including Artiles.

Those committees, in turn, have received money from, among other sources, Artiles’ own committee, Veterans for Conservative Principles, and from the campaign arm of the controversial Internet café industry that promotes small-time gambling storefronts.

Zapata, who has raised far more in campaign contributions than Machado, said he expects a new attack regarding his expunged 1992 arrest for possession of steroids, and his parents’ 1982 indictment and subsequent conviction for money-laundering. Zapata, who served eight years in the Florida Legislature, has never publicly spoken about his parents’ arrest.

As part of Operation Greenback, Ivan and Eugenia Zapata were convicted of funneling other people’s money — in some cases from drugs and other illegal activities — through fictitious Miami bank accounts to Panama.