This is where weather and electoral forecasts start to differ. For weather, we have fundamentals — advanced science on how atmospheric dynamics work — and years of detailed, day-by-day, even hour-by-hour data from a vast number of observation stations. For elections, we simply do not have anything near that kind of knowledge or data. While we have some theories on what influences voters, we have no fine-grained understanding of why people vote the way they do, and what polling data we have is relatively sparse.
Zeynep Tufekci, New York Times