Turnout in the first 29 GOP primaries – up to and including Indiana – averaged 16.6%, according to our analysis. But turnout in the final nine contests, after Trump became the presumptive Republican nominee, averaged only 8.4%.
By contrast, the heated battle between Hillary Clinton and Bernie Sanders engaged Democratic voters for longer. Less than a day before the final five states held their primaries, the Associated Press reported that Clinton already had clinched the nomination. But turnout in the five primaries held June 7 didn’t appear to be affected, averaging 14.1% compared with an average of 14.5% for the preceding 31 contests. (The District of Columbia will hold its Democratic primary June 14, but there likely won’t be enough voters there to significantly change the nationwide numbers.)
The overall Democratic turnout of 14.4% was well below the record 19.5% in 2008, but it was still the second-highest since 1988’s primary season.