I'm usually able to tell pretty early on election night (once some rural results come in) who will likely win. This was especially true in 2016 when I basically thought Clinton was doomed when comparing her rural performance to that of Obama. In terms of month, we usually have a pretty good idea by early November unless polls are very close in all the key states.
I remember watching the early FL results in 2016. One commentator was looking at how Hillary was doing better than Obama 2012 in Duval county, Orange county, Hillsborough county, Miami Dade and Broward counties. Trump's FL win was based on his blowouts in exurban central FL and SW FL.
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