Predicting Primary Success: Early-Cycle Fundraising and Establishment Support in Republican Senate Races

Although party primary elections select party nominees, voters must choose among ideologically similar candidates without the aid of party labels. Candidate-centered theories predict that early fundraising drives electoral success, while the “Party Decides” theory points to elite coordination via endorsements as the key influence on voters. We hypothesized that endorsements would predict vote share more strongly than fundraising once both were controlled for. Using 16 competitive Republican Senate primary races (2008–2022; 45 candidates, 13 states), we created endorsement scores weighted by endorser type, incorporated FEC fundraising data, and estimated OLS models with standard errors clustered by race. Endorsement accumulation had a large, significant effect on vote share (β = 0.67 percentage points per point, p = 0.003) across seven specifications. In contrast, early fundraising showed no independent effect once endorsements were controlled (β = 1.12, p = 0.564). But when looked at separately from traditional endorsements, Trump's coefficient was remarkable: 12.1 percentage points (p = 0.006), and each of the four candidates endorsed by Trump won their primaries. Extended analyses reinforced these findings: endorsements predicted outright victory, Trump's effect strengthened as field size grew, and in-state endorsements outperformed out-of-state ones. The Ohio 2022 polling results indicated that endorsements came before candidate momentum, rather than after, reducing selection concerns. Out-of-sample application demonstrated the model’s predictive validity, as successful predictions were made in three Republican Senate primary elections in 2024 in which the model was not trained. These results extend the “Party Decides” framework to Senate primaries for the first time and identify Trump as a uniquely powerful endorser whose influence exceeds standard elite coordination.