California's 10th congressional district

District borders, 2002-2012.

Prior to redistricting by the California Citizens Redistricting Commission going into effect in 2012, the 10th district stretched from Livermore to Dixon and the outskirts of Vacaville. It consisted of portions of Alameda, Contra Costa, Sacramento, and Solano Counties.

The 10th district received national attention in 1996 when Democrat Ellen Tauscher defeated incumbent Republican Bill Baker.[5]

In the 2002 redistricting of California, all seats were made safe[6] for the parties of incumbent officeholders. The district boundaries were extended to include parts of Solano County, southwestern Sacramento County, eastern Contra Costa County and El Cerrito in western Contra Costa County. Although much of the suburban Tri-Valley region was lost to the 11th Congressional District, the city of Livermore remained in the 10th at Tauscher's request (as a member of the U.S. House Committee on Energy and Commerce, she had some oversight responsibilities over the U.S. Department of Energy, and hence indirectly of Lawrence Livermore National Laboratory).

On June 26, 2009, Tauscher resigned her seat to be sworn in as Undersecretary of State for Arms Control and International Security.[7][8][9] In the ensuing special election held on November 3, 2009, former Democratic Lieutenant Governor John Garamendi won the seat over Republican David Harmer 53.0% to 42.7%. Immediately following redistricting, Garamendi successfully ran for re-election in California's 3rd congressional district, which shares many municipalities with the 2002 version of the 10th district (e.g. Vacaville and Fairfield), but lies significantly northwest of the current 10th district.

The current 10th congressional district includes all of Stanislaus County (including Ceres, Oakdale, Modesto, Riverbank, and Turlock) and the southern portion of San Joaquin County (including Tracy and Manteca).[4]

This went into effect in 2012, as the result of redistricting by the California Citizens Redistricting Commission. The new 10th includes much of the core of the old 18th district (Modesto, Ceres, and the southwestern half of Stanislaus County), though the 18th also included a substantial portion of Stockton. It also shares much of the northwestern portion of the old 19th district (Turlock, Riverbank, Oakdale, and the rest of northeastern Stanislaus County).

Republican Jeff Denham transferred from the 19th District to the new 10th. He held it for three terms until being defeated in 2018 by Democrat Josh Harder.

The California Secretary of State publishes reports on California voter registration on a regular basis. Before the 2018 primary election, they published a report dating May 21, 2018.

Republican John D. Fredericks won the special election to replace fellow Republican Henry Z. Osborne, who died in office. Data for this special election is not available.[34]

Democrat Alfred J. Elliott won the special election to replace fellow Democrat Henry E. Stubbs, who died in office. Data for this special election is not available.[42]