Just as voters in general elections have always been a distinct subset of the population in general; voters in primary elections have been a distinct subset of voters in the general election. Turnout in primaries, especially in midterm congressional primaries, has been infamous for the fact that it is so paltry. This can be attributed, in large part, to two factors: the fact that congressional primaries are often uncontested; and the fact that in states with party registration, it is difficult or impossible for independents to vote in a primary. But even contested primaries and open primaries suffer from low turnout. For instance, in the well-known race in New York’s 14th congressional district, where Alexandria Ocasio-Cortez defeated incumbent Joe Crowley, only 29,778 voters, or 13.9 percent of the 214,570 active registered Democrats who were eligible to turn out did so. Though these numbers are quite low, they are also familiar to students of congressional primaries.

While research from the Pew Foundation finds that primary turnout in 2018 was substantially higher than the last midterm election in 2014, it still remains well below turnout in other elections. A total of 36,700,000 primary votes were cast this year, which is still a fraction of the 138,846,571 votes cast in the 2016 presidential election (general) and the 83,262,122 votes cast in the 2014 midterm elections (general).1

Thus, in studying primary voters we are dealing with a small fraction of the population. And so, it is important to figure out who they are. We start with gender because that appears to be the most dramatic attribute dividing the two parties in 2018.

As Figure 2 below illustrates, Republican primary voters are primarily men and Democratic primary voters are primarily women. Furthermore, according to figures from the U.S. Census, the Republican Party is more male and less female compared to the average distribution of genders in the Republican sampled districts, whereas voters in the Democratic primaries are much more female than the district averages. (See the Appendix for further details on Census comparisons.)

Significant differences between each party’s primary voters arise when we look at race as well as gender. Figure 3 shows that almost half of the Democratic primary voters are non-white; while an overwhelming majority of Republican primary voters are white.

Consistent with prior research on primaries and on voters in general, we found that primary voters tend to be a very well-educated group—especially compared to the voting-age population in their districts. Figure 4 shows that there is very little difference between the parties on this characteristic; 62 percent of Democratic voters have a college diploma or more and 58 percent of Republican voters fall into those two categories. We can also analyze how this breakdown compares to the general population in those districts. Republican primary voters are twice as likely to have done postgraduate work as the voting-age population in their district; and for Democratic primary voters, the gap is even bigger. Although the gap is large in both parties, on the Democratic side, primary voters are almost twice as likely as the voting age population in their district to have college degrees or postgraduate study.

Of course, there is a relationship between education and income, so it is not surprising to find out that primary voters in both parties are not only better educated, but wealthier than the average of their respective districts. Figure 5 breaks out primary voters by their family’s income. Primary voters are richer than those who don’t vote in either political parties, with very few differences. Over half the primary voters in both political parties earn more than $75­,000 per year—median family income in 2018 in America is, according to the latest census data, $60,309 per year.2 As Figure 6 indicates, the differences are most stark at the extremes. Republican primary voters are 12 percent more likely to earn over $150,000 per year than the voting-age population in their districts; Democratic primary voters are 6 percent more likely to earn over $150,000 than the voting-age average in their districts.

In addition to being better educated and richer than their neighbors, primary voters in both parties tend to be older than their neighbors. However, important differences exist across the two parties. Figure 7 displays the age of party primary voters relative to the average age distribution in the general population of the districts sampled. While Democratic primary voters are somewhat evenly distributed across the three age cohorts, Republicans voters tend to be older than the general population. For example, in the districts studied, 18- to 39-year-olds make up about 37 percent of the population. This same age cohort makes up 30 percent of the Democratic primary electorate and only 19 percent of the Republican primary electorate. In the long run, the Democrats’ better showing among the youngest voters could bode well for the party if it stays popular with these voters as they age.

Religiosity has become another demographic that divides the two political parties. As Figure 8 illustrates, Republican primary voters are much more likely than Democratic primary voters to attend religious services regularly and Democratic voters are much more likely than Republican voters to attend religious services rarely or never.

Continue to the next section, “The ideology of primary voters.“