Many people tend to overlook that these days, voting begins several weeks before "election day." According to data compiled by political scientist Michael McDonald at electproject.org, nearly 40% of votes in the 2012 Presidential election were cast before election day. That total could be eclipsed this year, although some states have curtailed early voting for political reasons.
As of this morning, according to McDonald's data, more than 4 million people have already voted in this election. With early "in-person" voting starting in more and more states, that figure will go up rapidly over the next few days.
States vary widely in the data they report from the early ballots. No state, of course, reports on who the early voters cast their ballots for, but some do provide useful demographic information.
Some interesting tidbits from the data so far:
Iowa is a state where Donald Trump has consistently polled favorably. He holds a 3.7% lead over Hillary in the polling average for that state. But Iowans have been voting now for quite a few days, and the tilt is heavily Democratic. So far, 235,000 votes have been cast in Iowa (there were 1.5 million total in the state in 2012). Of those, 49% are from registered Democrats, and 32% from Republicans. In contrast, in 2012, 42% of absentee ballots were from Dems, and 32% from Republicans.
Of course, that doesn't mean Hillary is winning or Trump is losing. The data could (and likely will) change as additional ballots come in. Also, it may be that traditional blue collar white Democrats in Iowa are voting for Trump. So, we won't get too excited about the data so far. But, if I was Trump, I'd be worried--if he loses Iowa, he is likely to go down to a landslide defeat nationally.
North Carolina and Georgia provide a demographic breakdown of their early voting data. What's interesting here is the gender divide. In Georgia, 433,000 votes have been cast (3.9 million total votes in 2012). Of those, 56% are from women and 42% from men. (Nationally, women make up about 53% of all voters.)
In NC, 221,000 ballots have been cast, of which 55% are from women and 43% men.
Of course, this could just be that women are voting early, and men will catch up at some point. And it doesn't tell you who they are voting for.
Finally, in Virginia it is noteworthy that early balloting in Democratically strong Northern Virginia is up 56% over 2012. Virginia is probably a lost cause for Trump anyway--polling has consistently shown a race their that is not close at all.