Direct-mail pieces thrashing state Sen. Perry Thurston Jr., an incumbent in a crowded Democratic primary in South Florida, are linked to a web of political organizations with ties to a prominent Republican consultant.

The mail accuses Thurston, a likely future Senate Democratic leader, of being a closeted Republican and not a progressive, two claims Thurston said are insulting.

But such attacks could prove damaging in the heavily Democratic Senate District 33 in Broward County, particularly when the knock is peddled by a political committee named Progressives, a title that is generally associated with Democrats.

The practice of members of one party covertly interfering in another party's primary is not new, and it has become a growing trend in competitive races in Florida. In this year's election cycle, examples include Thurston's primary and another heated Senate race in Seminole and Volusia counties to replace term-limited Sen. David Simmons, R-Altamonte Springs.

"It's always the same reasons. If it is a competitive primary, you often want to weaken the candidate that is the front-runner to allow your own party's candidate to have a better chance at capturing that seat, or helping somebody who has more of a track record in crossing the aisle," Susan McManus, a retired political science professor at the University of South Florida, said.

In Thurston's case, he is being attacked by a political committee led by Republican consultant William Stafford Jones. The former Alachua County Republican Party chairman did not respond to requests for comment, but his ties to the committee are amplifying concerns among Democrats that Republican-linked money could be influencing Democratic primaries ahead of the Aug. 18 elections.

"Do I know this is Republican meddling in our primaries? Yes. Can I prove this is Republican meddling in our primaries? No. And that's the point because our campaign finance system is irretrievably broken," Beth Matuga, a longtime Democratic political consultant and fundraiser, said in an interview.

A large portion of the political committee's contributions have come from organizations that disguise donors by passing money from one political committee to another, making the money hard to trace.

One of the contributing committees, Foundation for a Safe Environment, is linked to Jones and GOP attorney Richard Coates. Jones chairs more than 100 political committees, according to state election records.

Matuga believes Republicans are inserting themselves into Thurston's primary to divert Democratic resources away from other contentious races as Democrats try to flip three seats in the November election to gain a 20-20 split in the Senate.

"The marginal dollars that we have to spend in defense in these primaries certainly is more impactful to us than Republicans, who have had much more funding over the years than we have," she said. "So, they know that if they get us to spend in the primaries, that it does have an impact on our ability to fully fund general elections."

Thurston is running in a safe Democratic seat, facing off against three other Democrats: Steven Meza, Terry Ann Williams Edden and Shelton Pooler. Republicans are not running a candidate, so the Democratic primary will decide the winner of the seat.

Thurston, who is slated to become the Senate Democratic leader in 2022 if he gets reelected to his seat, thinks he can fend off the attacks on his own, because he says they are laughable.

"I think that the fact that they are photoshopping pictures of Trump with me and saying that I vote with Republicans, I think that just goes a step too far," he said.

Farther north, the Senate District 9 battle to replace Simmons is growing more contentious, and Democrats are calling foul on what they say are Republican "dark money" tactics in the crowded Democratic primary.

Five Democrats are competing in the primary: Patricia Sigman, Rick Ashby, Alexis Carter, H. Alexander Duncan and Guerdy Remy. Republican candidate Jason Brodeur, a former state House member from Sanford, is unopposed in his primary.

Sigman was recruited by the party to run for the seat and Democrats are slamming Floridians for Equality and Justice, a political committee whose address is a UPS store in Miami, for attacking her. In a mailer, the committee, which has yet to disclose any contributions or expenditures, calls Sigman a "do-nothing mouthpiece."

In a separate mailer, paid for by the same organization, one of Sigman's Democratic opponents is lauded. The piece said Ashby is the "progressive choice for the state Senate" and showcases a photo of him with Sen. Annette Taddeo, D-Miami.

"I am outraged this organization would use my likeness to promote a candidate I do not support," Taddeo said in a statement. "Dark money attacks have no place in a Democratic primary, and a real progressive would never resort to using dark money to win an election."

While Democrats complain about Republican-backed committees spending money to influence Democratic primaries, incoming Senate President Wilton Simpson, R-Trilby, has accused Senate Democrats of meddling in a Republican primary in Southwest Florida's solidly conservative Senate District 27.

Simpson has questioned Republican candidate Heather Fitzenhagen's loyalty to the GOP and has accused her of putting the Republican majority of the Florida Senate in jeopardy. Fitzenhagen, a House member from Fort Myers, jumped into the Senate race on the last day of qualifying in June, without warning Republican leaders.

Simpson believes incoming Senate Minority Leader Gary Farmer, D-Fort Lauderdale, recruited Fitzenhagen to run against Ray Rodrigues, a former House majority leader who has the backing of Senate Republicans. Fitzenhagen has denied the accusations. Farmer has not responded to requests for comment.

In the 2016 election cycle, the Miami Herald reported that mailers paid for a political committee backed by Republican interests attempted to influence three contested Democratic primaries, including the successful election of then-Rep. Darryl Rouson of St. Petersburg to the Senate.

As Democrats decry the committees' attack tactics, they point to flaws in Florida's campaign finance system.

"The campaign finance system that we operate under and the way they are doing this is perfectly legal," Matuga said. "There's nothing illegal about it. If you have enough money and enough resources, you can do all of this legally."

The system is also making it more difficult for political analysts and voters to discern sources of attack ads, McManus said.

"The names of these PACs are designed to obfuscate or confuse people," she said. "So, most people are not going to go through the trouble to find out exactly who is funding. It's becoming more difficult for voters to really sort out who a PAC really is."

Ana Ceballos reports for the News Service of Florida.