popular vote vs referendum

popular vote

noun
  • The total number of votes of a given electorate. 

referendum

noun
  • A direct popular vote on a proposed law or constitutional amendment. The adposition on is usually used before the related subject of the vote. 

  • An action, choice, etc., which is perceived as passing judgment on another matter. 

How often have the words popular vote and referendum occurred in a corpus of books? (source: Google Ngram Viewer )