Proposition 40, known as the California billionaire tax, is polarizing, and to fight it two other propositions have been placed on the ballot.
Federal law enforcement officers said they are working to intercept the letters.
California Attorney General Rob Bonta slammed the letter.
Current federal law bans most robocalls to cellphones without prior consent.
Trump says the U.S. will obtain diesel from Russia, relaxing years of US pressure on Moscow to combat prices ahead of midterms.
An Arizona congressional candidate says she was knocked unconscious and sexually assaulted this summer while she was repairing campaign signs overnight.