State Representative David Linsky, a Natick Democrat who represents Natick, Millis, and Sherborn in the House, will face Republican William J. Callahan in the race for the Fifth Middlesex District seat on Nov. 6, after Callahan mounted a successful write-in campaign. Callahan, also a Natick resident, garnered 415 write-in votes in Natick in the Sept. 6 primary. In Sherborn, 63 people voted for him, and in Millis, he received 76 votes, according to town officials. Linsky has represented the Fifth Middlesex District since 1999.