Harry Enten in FiveThirtyEight (my emphasis):
He won the Michigan primary over Hillary Clinton, 50 percent to 48 percent, when not a single poll taken over the last month had Clinton leading by less than 5 percentage points. In fact, many had her lead at 20 percentage points or higher. Sanders’s win in Michigan was one of the greatest upsets in modern political history.
Both the FiveThirtyEight polls-plus and polls-only forecast gave Clinton a greater than 99 percent chance of winning. That’s because polling averages for primaries, while inexact, are usually not 25 percentage points off. Indeed, my colleague Nate Silver went back and found that only one primary, the 1984 Democratic primary in New Hampshire, was even on the same scale as this upset. In that contest, the polling average had Walter Mondale beating Gary Hart by 17 percentage points, but it was Hart who won by a hair over 9 percentage points.
What are we to make of this? Is Hillary more vulnerable than we had all assumed? Could Bernie Sanders possibly be the Democratic nominee? Could we be talking about President Sanders at this time next year?
While I'm still processing what happened last night, consider this: we still have "large delegate prizes left like California, Florida, Illinois, New York, New Jersey, Ohio and Pennsylvania." And unlike South Carolina, Alabama, Arkansas, Georgia, Tennessee, Texas, Louisiana and Mississippi -- all states won by Mrs. Clinton -- many of the upcoming contests are in blue or purple states, ones which may actually vote Democratic in the fall.
Everyone's been talking about the possibility of a contested Republican convention. Did we get the party wrong?