The United States is a maritime superpower, able to project power across the Atlantic and Pacific oceans. Losing the Pacific coast would end that. I can't imagine it being let go easily.
The bulk of U.S. naval forces in California are in the San Diego region. It's possible the U.S. could negotiate a border that leaves the naval bases as well as the Marine Corps base at Camp Pendleton as part of the United States. (The entire swath of Southern California below Los Angeles.)