Primaries might also contribute to party transformation by incentivizing candidates to move position within an election cycle. Candidates might face a “strategic positioning dilemma” if they must first satisfy an extreme selectorate to earn the nomination before facing a comparatively moderate general electorate. This chapter therefore tests whether all candidates in a primary adapt their positions away from the center during the nomination phase of a single election cycle, presenting general election voters with polarized choices. To scale positions both during and after a primary it uses a text-as-data approach based on candidates’ communication on Twitter during the 2020 election cycle. It finds that Democratic candidates who lost primaries became significantly more moderate immediately after their defeat, especially if they lost in ideological or factional primaries. It does not observe this pattern among Republican losers. This chapter demonstrates a further way in which primaries may contribute to polarization, incentivizing candidates to adopt positions further from the ideological center during the nomination phase of the election cycle.

This chapter presents the descriptive argument that primary elections have undergone a fundamental transformation along several dimensions between 2006 and 2020. In particular, it presents evidence that during this period primary elections became factional, with candidates receiving support from distinct parts of their party coalition; and ideological, with candidates framing their candidacies in terms of positional differences from their intra-party opponents. These changes were associated with a higher rate of contested primaries.

The book concludes by considering the broader implications of the findings of the previous chapters to the study of congressional primaries and the institution of Congress. This chapter advocates that we need to rethink how primaries matter in influencing candidate positioning and elite party identity. It also considers the implications for the scholarly community, citizens’ representation, and practical applications given the current focus on primary reform. It suggests several avenues for further research that can build on the book, as well as identifying potential limitations of this work. It concludes by considering the implications of these findings for the two parties in 2023 and beyond.

Having shown that primaries can reorient parties in Chapter 4, this chapter tests the first mechanism through which primaries are said to contribute to partisan polarization: the selective effect of voter preferences. It therefore tests whether primary voters prefer noncentrist candidates, all else being equal. Through a set of four analyses it tests whether primary voters prefer candidates further from the center when they are presented with a comparatively moderate and polarized alternative, whether moderate incumbents are more threatened, whether candidates who emerge from (ideological and factional) primaries are more “extreme” than other candidates, and whether there is any relationship between turnout and nominee position. Taken together, the findings in this chapter demonstrate the absence of a select effect from primary voters in nonincumbent primaries and only a weak and substantively small effect when an incumbent is present, suggesting that the polarizing effect identified in Chapter 4 is largely independent from the preferences expressed by primary voters.

The Introduction sets out the central puzzle that the book seeks to solve. Descriptively, it asks whether primaries have transformed in the twenty-first century by using a series of case studies to illustrate the central descriptive argument of change. It then frames the importance of the second half of the book, justifying the focus on elite partisan positioning and ideological change in relation to recent primary elections as a (potential) mechanism. It then clarifies the data collection process and sources used. Finally, it focuses on partisan differences between the Republican and Democratic parties before providing an outline of the book’s structure.

This chapter identifies why the descriptive changes observed in Chapter 2 took place. To answer this question, it proposes a model of structural change being responded to by the key actors during the nomination process. These structural changes are grouped into three categories: changing electoral incentives, new regulatory reforms, and technological developments. These changes elicited responses from actors within the party network in terms of organizational structure and electoral strategy, by candidates in terms of whether and how they ran for office, and among voters in terms of their participation and motivation in nationalized congressional primaries.

Part II of the book shifts in focus from primary transformation to party transformation. This chapter tests whether primaries can change parties under the most likely conditions: Republican factional primaries in the Tea Party era. Using a difference-in-differences design, this chapter shows that in this most visible case, pressure from the reactionary Republican faction in primary elections served to reorient the party rightwards. It therefore demonstrates that, under the right circumstances, primaries can contribute to partisan polarization in Congress.

Primaries may contribute to polarization in other ways than a selective effect emanating from voters. This chapter tests a second potential mechanism of polarization, where incumbent members of Congress may respond to being challenged in a primary by adopting more liberal or conservative voting patterns in subsequent congresses. To test incumbents’ responses, it uses a series of fixed-effects models, clustered at the representative level, with roll-call movement as the key dependent variable. When considering the universe of all primary challenges, incumbents do not respond positionally, but when primaries are ideological and factional, they move toward their ideological pole. These effects are larger for factional primaries, indicating that incumbents are most responsive when a primary opponent has the support of an alternative party faction. These effects are larger for Republican than Democratic members of Congress, which is one way in which primaries may contribute to asymmetric polarization. These findings indicate that primaries may matter for polarization because incumbents believe them to be important and so are responsive to them.