The Republican contenders for Suffolk district attorney and sheriff, the county’s marquee fall races, met for the first time Friday and agreed to make backroom endorsement deals between Democrats, Conservatives and the Independence Party a major thrust of their campaigns.

“I think it’s a big issue and there’s a synergy,” Larry Zacarese, the GOP sheriff primary winner, said after an hourlong session at District Attorney candidate Ray Perini’s Islandia campaign headquarters.

“Both our opponents have multiple party lines, and how they got them is a direct issue for the voting public that is tired of decisions being made by three men in a room,” said Zacarese, assistant chief of the Stony Brook University Police and a former NYPD sergeant.

“It has to be an issue in the race,” said Perini, who spearheaded the issue four years ago during a GOP primary against Democratic incumbent Thomas Spota. While Perini lost the race, Suffolk Republicans later barred GOP candidates from taking Democratic cross-endorsements in nonjudicial races.

Perini said “an eyebrow should be raised” any time there’s a pact between Conservatives and Democrats because their views vary so much. He pointed to Conservatives’ endorsement of county police Commissioner Tim Sini for district attorney, even though he also accepted the liberal Working Families ballot line.

Perini said Conservatives in the sheriff’s race first backed Republican state Sen. Phil Boyle, despite his lack of law enforcement credentials, to weaken Suffolk GOP chairman John Jay LaValle and because Boyle employs the wife of convicted former Suffolk Conservative leader Edward Walsh.

Perini said a still-unhatched part of the deal is Democratic backing for Suffolk Conservative chairman Frank Tinari’s wife, Marian, now a District Court judge, in the countywide race for Surrogate’s Court, a patronage-rich post.

Richard Schaffer, the Suffolk Democratic chairman, said there is no agreement on a Tinari endorsement, and called the Republican contenders “disingenuous.”

Schaffer produced emails showing Zacarese met with him at his law office on May 21, 2016.

“He said, ‘You should have your own sheriff candidate,’ ” Schaffer recalled. He said Perini also had intermediaries approach him about a possible endorsement — arguing that “Sini would be Bellone’s guy,” a reference to strains between Schaffer and Democratic County Executive Steve Bellone.

“Both of them are complaining and trying to give politics a bad name by lying to voters,” said Schaffer, “Both wanted to be a recipient of a cross-endorsement.”

Zacarese acknowledged meeting Schaffer “to pick his brain,” but said he couldn’t recall details of the meeting. Zacarese said his position always has been “cross-endorsements pervert the political process.” Perini called Schaffer’s assertions “absolutely untrue.”

How the issue will play in November is unclear. The Conservative ballot line typically draws 10 to 12 percent of the vote, but many hard-line party members are wary of Democrats. Newly energized Democrats and nonaligned voters upset with President Donald Trump may be irate about Democrats aligning themselves with Conservatives.

Michael Dawidziak, a consultant who works primarily for Republicans, said the cross-endorsement issue may be “too obtuse” for most voters.

“People don’t really understand backroom politics or pay attention to it,” he said. “They don’t want to know how the sausage is made.”

But Zacarese said he believes the issue will resonate as it did in the primary because “it makes the average voter scratch their head in disgust.”