In 1948, the headlines were “Dewey Wins”. However, the result was that Truman won.
See “Dewey Defeats Truman”, Wikipedia, https://en.wikipedia.org/wiki/Dewey_Defeats_Truman.
See”A ‘Dewey Defeats Truman’ Lesson for the Digital Age”, Jim Gutenberg, New York Times, 2016-11-09, http://www.nytimes.com/2016/11/09/business/media/media-trump-clinton.html;
“Case Study 2: The 1948 Presidential Election”, https://www.math.upenn.edu/~deturck/m170/wk4/lecture/case2.html
However, what really counted was how polls were taken, often by telephone. In 1948, many who leaned towards Truman did not have telephones and weren’t called. See “Dewy Defeats Truman, Joe Walter, “The True Facts, 2019-05-31, http://www.thetruefacts.com/dewey-defeats-truman/
So, were the predictions off because too many people don’t answer their cell phones if they don’t recognize the number? I know that I do.
Another thought, maybe many of Trump’s supporters who did answer their phones didn’t admit that they were Trump supporters.