The Clintons went undefeated in Michigan for 24 years. It started on March 17, 1992 when Bill Clinton won the Michigan primary by 25 points. In the general election, Clinton turned Michigan blue for the first time since 1968 and the state hasn’t voted Republican in a presidential election since.
Prior to Tuesday, polling averages said Clinton would win by more than 20 points. That, combined with 24 years of history, made it seem impossible for Sanders to win. But when the polls closed, the race wasn’t immediately called for Clinton. In the end, Sanders pulled off the biggest upset in primary polling history. At FiveThirtyEight, Nate Silver points out that the previous record was set in 1984, when Gary Hart overcame a 17 point polling deficit in New Hampshire to beat Walter Mondale. And you’d expect polling would be better today, 32 years down the road.