Bob Ray, president of the Democratic Action Club, said he arrived at the office Thursday and found "No Jew in the White House" had been written in ink in large letters across two large erasable boards and a planning calendar.
Police Lt. Mike Simpson said investigators were told a campaign worker left the office unlocked Wednesday night, and there were no suspects.
Ray said campaign workers plan to keep the message up to remind them of the importance of the election.
The office is a local organizing center for the presidential campaign of Vice President Al Gore and his running mate, Sen. Joseph Lieberman, who is Jewish.
Chico is 100 miles north of Sacramento.