Senate Race Tests California’s Primary System

SAN FRANCISCO—When Democratic California Sen. Barbara Boxer announced last year that she wouldn’t seek re-election after four terms in office, expectations were high that a number of prominent Democrats and Republicans would rush to compete for her seat.

Now, roughly a month before the state’s June 7 primary, there are few marquee names in the crowded field of 34 candidates. And the general election could be California’s first...