Cooper became the Labour Party's candidate from an all-female short list, in the constituency of West Lancashire at the 2005 General Election, following the retirement of the sitting MP Colin Pickthall. Cooper was first elected to the House of Commons at her fifth attempt and third party with a majority of 6,084. She made her maiden speech on 24 May 2005. In September 2005, Cooper, as part of the Labour Friends of Israel, made an official research visit to Israel. In September 2020, she was appointed a vice-chair of Labour Friends of Israel.