The Working Families Party has endorsed Democrat Laura Curran for Nassau County executive, dealing a blow to her primary foe Charles Lavine, who’d also sought the backing.

Long Island Progressive Coalition director Lisa Tyson, a Working Families leader, said Friday that both Curran, a county legislator from Baldwin, and Lavine, an assemblyman from Glen Cove, have progressive records — but that the party sees Curran as more viable in a general election.

Top Nassau Republicans are backing former State Sen. Jack Martins of Old Westbury.

“It was a very tough decision,” Tyson said. “But we need the strongest candidate against Jack Martins.”

The Working Families nod means that Curran, who is also supported by county Democratic leaders, would have the minor party’s ballot line in November even if she lost the Democratic primary.

Curran said: “I’ll continue my advocacy for shared progressive values that’ll help bring Nassau into the 21st century.”

Lavine spokesman Andrew Mulvey said: “Chuck will continue working to earn every vote possible so we can make Nassau County an even better place to live for middle class families and those striving to join the middle class.”

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Nassau Comptroller George Maragos is also seeking the Democratic nomination. Tyson said he didn’t seek Working Families backing.