Simpson's opponent, Idaho Falls attorney Bryan Smith, got an endorsement from the Club for Growth, a national organization that supports cutting taxes.
Smith had said Simpson was too liberal for Idaho, citing his October 2013 decision to end the 16-day partial government shutdown Republicans attempted to use to force an end to the federal Affordable Care Act.
Simpson, who was endorsed by the National Rifle Association, said he had a strong conservative voting record in his eight terms in office.
He pointed to a budget rider he inserted in April 2011 that stripped wolves of federal protections. The measure marked the first time since the passage of the Endangered Species Act in 1973 that Congress forcibly removed protections from a plant or animal.
Simpson will face Democrat Richard Stallings in November. Stallings, a former four-term congressman from Idaho, ran unopposed in the primary. With Idaho being a strong Republican state, Simpson is not expected to face a serious challenge from his Democratic opponent.
The 2nd District encompasses the eastern half of the state and is home to farms and ranches, the Idaho National Laboratory nuclear facility, and the churches and temples representing a strong presence of The Church of Jesus Christ of Latter-day Saints.
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