The field in the Oct. 4 special election to replace ex-Rep. Christopher Cox (R-Calif.) has been set.

It includes two leading Republican contenders and an intriguing conservative among the 20 candidates expected to appear on the ballot.

Cox resigned recently to become chairman of the Securities and Exchange Commission. His Orange County seat is reliably Republican, and GOP state Sen. John Campbell is the favorite in the special election. He is supported by California Gov. Arnold Schwarzenegger (R) and most of the state’s GOP establishment.

But in the all-party primary, which enables voters of all parties to select a candidate regardless of their party affiliation, former state Assemblywoman Marilyn Brewer, who is also a Republican, has a legitimate shot as well. Brewer, a moderate, hopes to attract Republicans, Democrats and independents to her side in a race with no top-tier Democrat on the ballot. She is touting an endorsment from Arizona Sen. John McCain (R).

Jim Gilchrist, who is running on the American Independent Party line, could be a factor as well. He is a leader in the Minuteman Project, the controversial group that has established armed citizens’ patrols along the U.S.-Mexico border. Immigration is sure to be a top issue in the election, and Gilchrist could receive substantial attention — and conservative support.

If no candidate surpasses 50 percent of the vote on Oct. 4, the top finishers from each political party will advance to a runoff on Dec. 6.