Some 9.8% of voting-age adults participated in GOP primaries this year, the highest percentage in a midterm election since 1970 and well above the 7.3% who cast ballots in the 2006 midterm, the study found. Some 8.2% of voting-age adults took part in Democratic primaries, down from 9% in 2006 and the lowest percentage of all midterms since 1930.