 During his ten years representing the 80th Assembly District in the state legislature, Klein served as Chairman of the Subcommittee on Crime and the Elderly, the Committee on State-Federal Relations, and the Committee on Oversight, Analysis and Investigations.In 2004, after former State Senator Guy Velella was convicted on corruption charges, Klein declined to seek reelection to the Assembly in order to run for Velella's seat in the New York Senate.In the Democratic Senate primary, Klein defeated then-Assemblyman Stephen B. Kaufman.