Further to the comment that this election has in many ways already started, Michael McDonald of Elect Project posted the early vote numbers in a couple of states, as a percentage of total 2016 turnout:
https://mobile.twitter.com/ElectProj...12841176657921
Not all of them are swing states, but notably early vote in Wisconsin is already 16.4% of 2016 turnout, and in Minnesota it’s 11.3%.
Across the US, over 3.3 million people have already voted by mail or in person, and early voting hasn’t even started in some of the most populous states (like California).