A special congressional election has been called for Oct. 4 in a Southern California district to fill a vacancy created when Rep. Christopher Cox (search) resigned to become head of the Securities and Exchange Commission.

Gov. Arnold Schwarzenegger issued the proclamation for the election Monday.

State Sen. John Campbell (search) and former Assemblywoman Marilyn Brewer (search), both Republicans, have said they plan to run for the seat in the heavily Republican 48th District in Orange County.

Campbell, who has the governor's endorsement, is considered by many to be the front-runner. But Brewer has said she will soon announce endorsements from other key GOP leaders.

Former Rep. Robert K. Dornan (search), 72, the outspoken conservative who represented Orange County from 1985 through 1996 as a Republican, is considering a run as a candidate for the American Independent Party.

Unless one candidate receives a simple majority, a runoff would be held Dec. 6 among the top finishers from each party. Seven parties are recognized in the district.