The 2006 United States Senate election in Pennsylvania was held on November 7, 2006. Incumbent Republican Rick Santorum ran for re-election to a third term, but was defeated by Democrat Bob Casey, Jr., the son of former Pennsylvania governor Bob Casey Sr. Casey was elected to serve between January 3, 2007 and January 3, 2013. Santorum trailed Casey in every public poll taken during the campaign. Casey's margin of victory (nearly 18% of those who voted) was the largest ever for a Democratic Senate nominee in Pennsylvania, the largest margin of victory for a Senate challenger in the 2006 elections, and the largest general election margin of defeat for an incumbent U.S. Senator since 1980. Casey was the first Pennsylvania Democrat to win a full term in the Senate since Joseph S. Clark Jr. in 1962, and the first Democrat to win a Senate election since 1991. He was the first Democrat to win a full term for this seat since 1940. As of 2022, this was the last time the following counties have voted Democratic in a Senate election: Greene, Washington, Westmoreland, Somerset, Lawrence, Mercer, Armstrong, Indiana, Cambria, Warren, Forest, Elk, Clearfield, Clinton, Schuylkill, Columbia, and Carbon. Democratic primary The Democratic primary was held May 16, 2006. Candidates Bob Casey, Jr., State Treasurer, former State Auditor General, candidate for Governor in 2002 and son of former Governor Bob Casey, Sr. Chuck Pennacchio, University of the Arts history professor Alan Sandals, attorney Declined Barbara Hafer, former State Auditor General and State Treasurer Joe Hoeffel, U.S. Representative Results Casey won a landslide victory in the primary. Republican primary John Featherman, who ran against Santorum in 2000 as a Libertarian, had been expected to challenge him in the 2006 Republican primary. However, Featherman withdrew his candidacy after a GOP petition challenge because he did not have the necessary number of signatures to get on the ballot. As a result, Santorum won the Republican nomination unopposed. Candidates Rick Santorum, incumbent U.S. Senator and former U.S. congressman for Pennsylvania's 18th congressional district (1991–1995) General election Candidates Major Bob Casey, Jr. (D), State Treasurer Rick Santorum (R), incumbent U.S. Senator Minor Carl Romanelli (G), rail industry consultant, and was removed from the ballot by a Commonwealth Court judge on September 25, 2006 following a challenge from Democrats for failing to collect enough valid signatures required of third-party candidates. He lost the appeal to the state