There are a few possible explanations for the huge polling miss in the Michigan Democratic primary: For instance, if a significant number of Clinton supporters stayed home out of complacency or crossed over to the Republican primary to oppose Trump, that may have contributed to Sanders’s shocking win. We might get more explanations from pollsters soon — I’ve emailed several who showed Clinton ahead by 10 percentage points or more to ask why they think the polls were so far from the voting results.
Here’s another possible explanation: The most recent Michigan polls in our database stopped contacting voters Sunday, the night of the last debate, held in Flint, Michigan. Although many thought Clinton performed better than Sanders in the debate, perhaps voters felt differently. After many pollsters missed Ted Cruz’s Iowa win and suggested that Clinton would win Iowa easily — she won so narrowly that some call it a tie — several pollsters told us a lesson they learned: “Keep on contacting voters as late as possible.” But it’s up to poll sponsors to pay to contact voters until the final days of the race, and none did so in Michigan.