WASHINGTON — Lefty influencer Jack Schlossberg’s famous family lineage won’t be enough to win the competitive race to succeed outgoing Democratic Rep. Jerrold Nadler, according to longtime Kennedy biographer Laurence Leamer.

“First of all, the Kennedy name is not magic any longer,” Leamer, author of the upcoming book “Lady’s man: the careless life of John F. Kennedy,” told The Post.

Schlossberg, 32, the grandson of John F. Kennedy who is famous for his online trolling and incendiary screeds against Republicans, launched his campaign for New York’s Manhattan-based 12th Congressional District seat on Wednesday.

Part of his pitch to voters leaned on the Kennedy name, including with a photo on his campaign website in which he mimics poses with a bike in a way that is eerily similar to

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