How could so many be so wrong in predicting Trump vs. Clinton?

This year’s U.S. presidential election has been called one of the biggest political upsets in history. Just weeks ago, Hillary Clinton seemed headed for a big win. And though the race tightened in the lead-up to November 8, almost every respected election forecast had her as a strong favorite over Donald Trump. Betting markets were predicting Clinton as well. And so was Illinois political scientist Brian Gaines, an expert on polling and public opinion.