You don't seem to understand the California Jungle/Blanket primary system... All candidates from any/all parties run against each other in the primary, and the top two are the contenders for the general election. Recently this has led to two Democrats always being facing each other in the general election.

In her 2018 race, Feinstein faced another Democrat, who lost by 10 points in the general with Feinstein winning the vast majority of the vote in densely populated areas: https://en.wikipedia.org/wiki/2018_United_States_Senate_elec...