With nearly all districts reporting, Zeldin, a current state senator, leads George Demos, a former federal prosecutor, by a margin of 9,599-5,787.
Zeldin, of Shirley, is a U.S. Army veteran and served in Iraq. He is currently a lawyer and a major in the U.S. Army Reserve.
Zeldin's primary victory sets up a rematch with six-term incumbent Rep. Tim Bishop on Nov. 4. Zeldin lost to the Democrat Bishop in 2008.
The 1st Congressional District includes the East End, the Town of Brookhaven and parts of the Town of Smithtown.
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