California has long been the only state that bans all candidate withdrawals.  This policy has been in effect since 1917.  However, on August 22, the legislature unanimously passed AB 1784, which permits candidates in primaries (for non-statewide office) to withdraw during the filing period.  The bill also prohibits anyone from filing for two public offices in that same primary.  Thanks to Eric Wong for this news.

The bill has not been signed yet, but it seems obvious that it will be signed.  No legislator voted against it and no one testified against it.  The bill does not apply to the statewide state offices, nor to U.S. Senator.