Democrats were writing obituaries for California's GOP after winning a supermajority in the state legislature last November, thus gaining veto-proof power to raise taxes. But their legislative lock may have slipped after this week's special election in which Republican farmer Andy Vidak appears to have defeated a Democrat—in a heavily Democratic senate district—who had championed high-speed rail and a higher minimum wage.

If Mr. Vidak wins an outright majority—late Friday, he led with 49.8% of the vote and provisional ballots...