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The demographics of the turnout are going to be key. I have always doubted that Hillary is going to get out the yute vote, the African-American vote, and the millennial vote like Obama did. She is not going to be able to reassemble the Obama coalition.
If that is true, then the electorate is going to be more Republican. Every pollster is assuming that the makeup of the electorate will be the same as in 2008 and 2012. If not, then it everything is out the window.
In sum, it is who gets out to vote that will decide this election.