Answer: Four, although the first was the most watched and the most memorable. The first debate was held on September 26, 1960.
Read more about presidential debates in the Smithsonian article: Eight Lessons for Presidential Debates (October 2, 2012) and in Don’t Know Much About® the American Presidents.
Don’t Know Much About® the American Presidents (Hyperion Paperback-April 15, 2014)
