The low turnout comes amid high stakes. Republicans are driving for the six-seat gain required to control the Senate.
Nonetheless, Democrats saw a 29 percent decline from 2010's primaries, the 11th consecutive midterm elections to see a drop in participation.
Republicans posted a 15 percent decline in participation from 2010. But their rate was closer to historical norms after tea party enthusiasm in 2010 led to a turnout spike.
The two parties' combined participation rate this year — 14.8 percent — is less than half of the most recent high of 32 percent, posted in 1966.
In all, the numbers suggest the campaigns, party committees and independent super PACs are spending potentially record sums to reach fewer voters than ever before. With money washing through the system at staggering rates, each vote's value is skyrocketing compared to previous cost-per-ballot figures.
A major contributing factor is congressional redistricting that followed the 2010 census, the report suggests. Lawmakers drew district lines to favor one party, thus leaving the minority party unmotivated to show up at the polls to pick a nominee that was all-but-certain to lose in November.
"Organized interests representing the views of only 3 to 4 percent of the electorate can win those primaries and propel one of their own to office," the report's author Curtis Gans wrote. "The result can be seen in how the tea party has become a dominant player in Republican congressional and state legislative politics. But the danger exists for both parties."
That intra-party fighting among Republicans, however, produced a record-high for GOP turnout in specific primaries. In Oklahoma, the contest between tea party-backed T.W. Shannon and second-term Rep. James Lankford drove primary turnout to a record level of almost 10 percent of those eligible to vote in the primary for that open seat.
And in Mississippi, where tea party favorite Chris McDaniel attempted to deny six-term Sen. Thad Cochran another term, turnout reached 17 percent for their runoff.
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