On March 7, 2017, Cox announced his candidacy for Governor of California in the 2018 election. He garnered the support of 55% of Republican delegates at the spring 2018 California Republican Convention, but fell short of the 60% needed for the party's endorsement. Cox obtained the endorsements of Newt Gingrich and Donald Trump, though he tried to distance himself from the Trump endorsement. Cox also received endorsements from eight Republican members of Congress, including House Minority Leader Kevin McCarthy, Devin Nunes, Jeff Denham, and Mimi Walters, as well as by the National Right to Life Committee.