Well unfortunately the answer to that question is a rather dull one. The cheapest presedential campaign was the first two, of George Washington. Since he run unopposed in both his elections his campaign budget was practically zero dollars. So basically the first election was and will remain the cheapest one ever done. For a more modern election Trumans 1948 campaign was relatively cheap with not a great deal of Radio or Newspaper spots since he mostly ran on a barn storming campaign with a great deal of speeches on the do nothing congress.

source

The Last Campaign How Harry Truman won the 1948 election.

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