Gov. Arnold Schwarzenegger (searchwill endorse former California Secretary of State Bill Jones (searchin his bid to challenge Democratic Sen. Barbara Boxer (search), sources close to the governor said.

Jones, a 54-year-old Fresno farmer who spent more than two decades in public life before term limits forced him to step down in 2002, is the best-known candidate in a four-person field vying for the Republican nomination.

Other Republicans in the running are former U.S. Treasurer Rosario Marin (search), former Los Altos Hills Mayor Toni Casey and former AssemblyJones lost the race for the GOP nomination for governor in 2002 and actively supported Schwarzenegger during the gubernatorial recall campaign that put the actor in office last fall. He also served on Schwarzenegger's transition team.

Boxer, a liberal Democrat seeking her third term, publicly opposed the recall.

Schwarzenegger has hinted he was likely to support Jones, but the timing of the endorsement, expected Friday, surprised some observers, who believed Schwarzenegger would only get involved in the final days before the March 2 primary, if at all.

"We're once again seeing Gov. Schwarzenegger being a different kind of governor by taking a position in a primary -- and so shortly after he was elected himself," GOP consultant Arnold Steinberg said.