State Representative David Paul Linsky, a veteran legislator who has represented Natick, Sherborn, and Millis on Beacon Hill since 1999, is facing a challenge in the Fifth Middlesex House District from Republican William J. Callahan, a retired colonel who served two tours of duty in Iraq.

Callahan, 52, who retired from the Army National Guard at the end of June, said military rules prevented him from “actively engaging in partisan politics,” while on active duty, so he was unable to get the signatures necessary in time for his name to appear on the Republican state primary ballot in September.