An Institute of Governmental Studies poll of the California gubernatorial election of 2018, released June 8, shows these results: Gavin Newsom 22%, Antonio Villaraigosa 17%, John Cox 9%, David Hadley 7%. Newsom and Villaraigosa are Democrats and both have declared. Cox and Hadley are Republicans. Cox has declared his candidacy, but Hadley so far has only said he is exploring running. He is a former Assemblymember.
Even though June 2018 is a year away, these results show the potential for a November 2018 race between two Democrats, with voters not being permitted to vote for anyone else in the election itself. That would resemble the 2018 California U.S. Senate race, which was also between two Democrats.