The Suffolk County Deputy Sheriff’s Police Benevolent Association endorsed Republican Larry Zacarese on Sunday, the first union backing received by the assistant police chief of Stony Brook University in the race.

PBA president John Becker called it a “difficult decision” between county sheriff contenders Zacarese, deputy police chief at Stony Brook University, and Democrat Errol Toulon Jr., a former New York City Corrections officer and deputy commissioner.

“I think the fact is his heart’s been in it from the beginning,” Becker said of Zacarese after the endorsement was announced at a Smithtown event. “I think he’s really touched a nerve with the voters of the county. He’s truly transcended party lines.”

Toulon entered the race after last month’s primary, when state Sen. Phil Boyle (R-Bay Shore) dropped out after his defeat by Zacarese in the Republican primary.

Although all the major county unions had backed Boyle — as had the Conservative and Independence lines — Zacarese said he received support from rank-and-file law enforcement.

“This builds on tremendous momentum we have from the primary,” he said of Sunday’s endorsement.

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Toulon said in a statement, “As the most experienced candidate in this race, with a 25-year background in law enforcement, I will work cooperatively with all members of the Sheriff’s Office, whether or not they endorse me.”

The sheriff’s responsibility includes overseeing Suffolk County jails.

The Suffolk Deputy Sheriff’s PBA, which represents 260 members, had endorsed Boyle.

Toulon, a member of the Suffolk County Water Authority, also has Conservative and Independence parties’ backing, Democratic Chairman Rich Schaffer said.

Becker said Zacarese’s primary victory was a rebuke against the cross-endorsement system. “People are tired of it,” he said.