Just how dense, credulous and naive does Rep. David Rivera think voters are in the 25th Congressional District? The answer is very, based on the evidence from his latest caper. Would Rivera have tried this stunt — secretly helping to manage and possibly funneling money to a Democratic candidate in the August primary — if he didn’t think so?

Some older Cuban-American voters in the district, whom Rivera has assiduously courted over the years, may be forgiving, but even they have limits. And with this latest episode, Rivera is pushing the viejos and everyone else to the limit and, possibly, beyond.

Let’s be clear: David Rivera is a shrewd, clever, likeable guy whose entire life is politics. But it increasingly looks like he’ll have to find another profession. The FBI, Miami-Dade Police public corruption unit and U.S. Attorney are hot on Rivera’s trail, and this time may catch him. Last year, Rivera came this close to being criminally charged in a 52-count complaint for financial shenanigans, most stemming from a $500,000 fee he earned from a local dog track for getting gambling approved.

The money, however, somehow went to a company run by his mother and her partner; they subsequently gave Rivera a sizable “loan,” which he did not initially report on his financial disclosure. But the statute of limitations had run on most of the alleged offenses, plus the clever congressman found an election-law loophole that made it legal for him to accept and spend thousands of dollars on his campaign for state GOP committeeman from Miami-Dade. Very cagey. Also very lucky.

Now, it looks like his luck may have run out. Rivera appears to be up to his neck in a scheme to run a faux candidate against his old political nemesis, Joe Garcia, a Democrat he defeated before for a House seat. Putting up stalking horses is not new, but they’ve got to follow all the rules like real candidates, including reporting how much they raised and spent on their campaigns and where the money came from. It looks like Justin Lamar Sternad of Cutler Bay did not.

A federal grand jury will be hearing testimony from several political vendors, veterans all of previous Rivera campaigns, who provided Sternad specialized lists of voters, printing and mailing services, the expense of which he initially failed to report to the Federal Election Commission. As various election-law experts have noted, candidates who don’t tell the truth on those campaign reports don’t get a do-over. There are no mulligans on omissions as glaring as Sternad’s.

He initially reported spending nearly $11,000 on his campaign (the filing fee alone was more than $10,000) and had just $120 left in late July. Then The Miami Herald’s Marc Caputo and El Nuevo Herald’s Manny Garcia started looking at Sternad more closely and discovered he’d spent many thousands more dollars — much of it in $100 bills stuffed in envelopes — for printing and mailing services than he’d reported. Further, some of those vendors said they’d been hired by Rivera himself. Hugh Cochran, a former FBI agent who runs Campaign Data, told me he’d wondered why Rivera had ordered up voter lists for Democrat Sternad. He sent Rivera an email to that effect.

When I recently asked Rivera why he and his campaign had helped a Democrat in the primary, Rivera would say only that he had “issued a statement” about not knowing Sternad and was now “moving forward.” He sounded very much like a guy who had lawyered up.

He needs to. The grand jury is said to be hearing testimony from the political vendors, none of whom has broken the law or has anything to hide. Justin Lamar Sternad won’t answer his door or respond to calls and emails, and his attorney refuses to comment, too.

Sternad is 35, the father of five and a hotel bookkeeper. He reported earning $30,000 last year and yet his amended report to the FEC says he loaned his campaign nearly $53,000. My hunch is that Sternad, a political neophyte, was talked into running for Congress as a lark by someone who promised to pay all his fees and expenses. His campaign, as it happens, was managed by a political consultant named Ana Alliegro, who is a very close friend of — you guessed it — David Rivera.

He’s got some explaining to do. Maybe to the grand jury, FBI and U.S. Attorney. Certainly to the voters of the 25th Congressional District.