I have done some work comparing to 2014. I will post more in about an hour, but these numbers are not good at all.
I have done the comparison also. It is not an apples to apples comparison. Every election cycle going back to 2010, absentee ballots consume a larger and larger share of the electorate compared to early voting and election day voting.
Before it was primarily GOP votes used to using absentee voting - now it’s Dems and indies also using this voting.
This of course would necessarily affect ratios of who’s voting.
The sheer volume of absentee voters this election cycle makes moot any comparison to 2014. It appears that the usual early voting Dems are now using absentee ballots instead. This will be confirmed once early voting starts next week.
It is next to impossible to compare 2014 to 2018 and it’s even difficult to compare to 2016.
Iowa on the other hand - it is rather easy to compare to 2012, 2014, and 2016.