After the shocking result of the 2016 election, election data science was on its heels.
The polls almost all showed Hillary winning. How did Trump pull it off?
Because the Electoral College. The polls weren’t wrong; they predicted a 3% Hillary national popular vote win and were only off by 2; they just missed a few key states.
The primary problem is relying on national polls to predict an election that isn’t nationwide. The Electoral College system means that states elect the president, not voters nationwide. Hillary Clinton won the nationwide vote, but she lost the election, primarily because she ran up her margins in diverse major urban areas but lost almost the entire rest of the country.