How swing model assumptions shape vote-to-seat predictions

Most democracies distribute parliamentary seats based on electoral districts. Due to a lack of district-level polls, forecasters project national voting trends to district-level outcomes – drawing substantial public attention from voters. We know little about how different assumptions about these models influence forecasts. We address this gap by comparing uniform and proportional swing models, alongside additional variants. Using data from the past eight German federal elections, we assess predictive performance and complement the analysis with a simulation study. The findings from Germany demonstrate that while differences between swing models are generally modest, model choice affects the precision of predictions. The simulation further reveals that uniform swing performs better with larger national swings, lower volatility between districts, and more parties.