Click on your state to see how your primary works. California’s seems especially pointless:
Modified closed primary
After a 2000 U.S. Supreme Court ruling declared the state’s open primary system
unconstitutional, California set up a “modified” closed primary. A voter can mar
k any party’s primary ballot, but only votes cast by registered party members co
unt toward the official tally.
Web link of note: CNN: How does the primary work?
(At http://www.cnn.com/ELECTION/2004/special/president/primary.explainer/frameset.ex
clude.html)