The reason was unclear. Donnelly, a state assemblyman from Twin Peaks, did not immediately respond to a request for comment. Vassar, who went to work for Donnelly last year, declined to comment.
Donnelly, the Legislature's most outspoken gun rights and anti-illegal immigration advocate, leads all Republicans in recent polls in an uphill effort to unseat Gov. Jerry Brown.
Donnelly's former campaign manager, Jennifer Kerns, announced last month that she had quit his gubernatorial campaign, while Donnelly called her departure a "mutual" decision.
In an email Wednesday, Kerns said Vassar's firing "represents a continuation of poor judgment" by Donnelly.
PHOTO: Republican gubernatorial candidate Tim Donnelly speaks with reporters at the California Republican Party's biannual convention in Burlingame on March 15, 2014. The Sacramento Bee/David Siders