Former Suffolk County Legis. Steve Stern won a special election Tuesday to become the first Democrat in 40 years to represent the 10th Assembly District in Huntington. Stern, 49, of Dix Hills, defeated Huntington attorney Janet Smitelli 59 percent to 41 percent, with all precincts reporting. He will fill the seat vacated by Republican Chad Lupinacci, now Huntington Town supervisor. “This is an important win for us locally. I also strongly believe it makes residents of the 10th Assembly District part of the national story,” Stern said in an interview late Tuesday, referring to a possible Democratic advantage in November. “It was a great win. I attribute it to tremendous volunteers and a positive message.” Republican Doug Smith won the special Assembly election in the 5th District and Republican John Mikulin won in the 17th District in Nassau.