Evidence Pack: Do Nonpartisan Open Primaries Actually Reduce Political Polarization?

For decades, political polarization in the United States has been treated as a cultural inevitability—a symptom of a divided public, nationalized media, and geographic sorting. But a growing consensus among political scientists and election reformers points to a more mechanical culprit: the partisan primary system. Because primary elections typically draw low turnout dominated by the most ideologically committed voters, candidates are heavily incentivized to cater to their party's fringes rather than the broader electorate.[8]

To solve this "primary problem," a handful of states have fundamentally rewired how they nominate candidates, replacing closed, party-run primaries with nonpartisan open primaries. In these systems, all candidates, regardless of party affiliation, appear on a single ballot. Every registered voter can participate, and the top vote-getters—usually the top two or top four—advance to the general election. The theory is straightforward: if politicians must answer to the entire electorate rather than just their party's base, they will govern more collaboratively.[7][8]

As the 2026 election cycle accelerates, with several states debating ballot measures to adopt similar reforms, researchers finally have enough longitudinal data to test whether these systems actually work. The evidence, drawn from over a decade of elections in California and Washington, and newer data from Alaska, suggests that nonpartisan primaries do measurably reduce legislative extremity, though the transformation is gradual rather than instantaneous.[1][3]

The most comprehensive evidence comes from the USC Schwarzenegger Institute, which analyzed the voting behavior of U.S. House members over a 15-year period. Using DW-NOMINATE scores—a rigorous academic metric that maps a legislator's ideology based on their actual roll-call votes, rather than their campaign rhetoric—the study compared lawmakers elected in top-two primary states against those in closed primary states.[1]

The USC findings were unambiguous. Lawmakers elected under top-two systems cast significantly less extreme ideological votes than their counterparts in closed systems. Crucially, this moderating effect was nearly double the effect seen in standard "open" partisan primaries (where voters can choose which party's ballot to pull, but the ballots remain segregated). The research demonstrated that the structural shift of putting all candidates on one ballot forces a tangible change in how those candidates legislate once in office.[1]

California provides the clearest long-term case study. After adopting the top-two primary in 2010, the state's political trajectory diverged from national trends. A 2023 analysis by the Unite America Institute found that during the decade following the reform, the California legislature depolarized. This occurred during a period when nearly every other western state, and the U.S. Congress as a whole, became measurably more polarized.[3]

However, institutional skeptics caution against viewing nonpartisan primaries as a silver bullet. Research published by Cambridge University Press and analyzed by the University of Chicago's Center for Effective Government notes that the effects can be inconsistent. These researchers point out that California's depolarization coincided with the adoption of an independent redistricting commission and changes to term limits, making it difficult to isolate the top-two primary as the sole cause of the state's political cooling.[4][5]

The UChicago analysis also highlights a mechanical reality of the top-two system: most first-round elections still result in a traditional Democrat-versus-Republican matchup in the general election. While the system allows for same-party matchups (e.g., two Democrats facing off in a deep-blue district), these remain relatively rare. When they do occur, however, out-party voters (like Republicans in a Democrat-only race) tend to vote for the more moderate candidate, successfully pulling the winner toward the center.[5]

Alaska's recent adoption of a "Top-Four" primary combined with ranked-choice voting in the general election offers a newer, highly scrutinized model. First implemented in 2022, the Alaskan system was designed to give voters even more choices while retaining the moderating incentives of a nonpartisan primary. Early assessments published in the California Journal of Politics and Policy indicate the reform was largely beneficial.[2]

The Alaska study found that the new rules promoted more accommodative campaigning. Because candidates in the general election needed to secure second- and third-choice votes from their opponents' supporters to win a ranked-choice tabulation, they were financially and strategically incentivized to avoid scorched-earth attacks. The result was a campaign environment that rewarded coalition-building over base-mobilization.[2][6]

Furthermore, the Alaska data showed that while the open primary allowed more extreme candidates to make it onto the ballot, the eventual winners were more likely to be centrists willing to work across the aisle. State legislators from across the political spectrum have noted that the system provides them with "political cover" to negotiate bipartisan compromises without the constant threat of being ousted by a radical challenger in a low-turnout closed primary.[2][6]

This concept of "political cover" is central to the reform's success. In a closed primary system, a lawmaker who votes for a bipartisan compromise risks facing a primary challenger funded by ideological purists. Because the primary electorate is small and highly partisan, that challenge is often fatal. In a nonpartisan open primary, the lawmaker can defend their compromise to the entire district, relying on independent and moderate voters to survive the primary round.[7][8]

The compounding nature of these reforms is perhaps the most encouraging finding for democracy advocates. The USC study noted that the moderating effect was evident among incumbent lawmakers who adapted to the new rules, but it was even stronger among newly elected members who had never run under the old closed system. This suggests that as the "rules of the game" change, a different profile of candidate—one more inclined toward governance than grandstanding—chooses to run for office.[1]

While nonpartisan open primaries cannot single-handedly erase decades of cultural division, the evidence pack is clear: institutions matter. By aligning the electoral incentives of politicians with the preferences of the general electorate rather than the partisan fringes, these structural reforms offer a proven, mechanical pathway to a more functional and representative government.[1][3][8]