An election puzzle is that absentee ballot return rate by Republican voters is so far only 9.0% compared with 15.2% for Democrats, a relative deficit of 41% for Republicans.
There is some causal effect now reducing Republican voting in the 2016 primary compared with past elections. This should be no comfort to Democrats. It may not be a momentum issue. Republican voting may bounce back. November 8 will be a whole new election.
I speculated the 41% deficit in Republican performance might be due to inherent differences in absentee voting rates by party, or overperformance in districts with hot Democratic contests, or regional postal delivery speed affecting Republican areas. But not so. When I controlled for city, Republican underperformance was countywide. The effect of hot elections on absentee ballot return rate was noticeable in only two districts.
As proof of Republican underperformance in the 2016 primary, I controlled for voter earliness by averaging the day each voter voted early, whether by absentee ballot or at an early voting location, in four past elections. The only known bias was due to lack of earliness data for voters who were not registered in 2012 or 2014 elections. Absentee ballot return rate was plotted by party, Democratic, Republican, and Independent (other parties and no party affiliation), for average earliness day 29 to 0 in past elections, on a probability scale (see graph, above).
Absentee ballot returns in the 8/30/2016 primary election are strong, 19,447 votes cast out of 158,190 active requests, as of 8/5/2016. In the 2012 primary, 42,244 votes were cast, so 2016 is slated to dwarf 2012. Based on past elections, a little less than half of absentee ballot requests will be voted in the primary.