Election 2020: can the Democrats win the Senate?

2020 US Election Senate Control and Forecasting Methodology

• Senate Control Dynamics

  • Democrats hold a potential opportunity to regain control of the Senate after five years of Republican majority.
  • Democrats currently defend 12 seats, while Republicans defend 23, out of 35 total seats up for election.
  • Democrats require a net gain of at least four seats to secure a majority; this threshold drops to three net gains if Joe Biden wins the presidency and casts the tie-breaking vote as Vice President.
  • Historically, divided government (e.g., Republicans controlling Congress while Obama held the presidency) has significantly stalled legislative agendas; the article notes no major legislative achievements for six years post-2010.
  • Senate Majority Leader Mitch McConnell indicated that blocking Democratic motions was a core strategy during the final two years of the Obama administration.

• Forecasting Model Specifications

  • The Economist's data model calculated a 71% probability of a Democratic Senate victory as of October 6, 2020.
  • The prediction engine integrates two primary data sources: real-time polling data and historical "fundamental" factors.
  • Fundamental factors used include state voting history, incumbent status, candidate political experience, and campaign fundraising totals.
  • The model utilizes a training process where it tests variables against past election data (excluding a specific test year like 1976) to identify and remove "overfit" variables and shrink effect sizes to optimize accuracy on unseen data.
  • Projections combining polling with fundamentals demonstrate greater stability than polling alone, as evidenced by the consistent performance of the model during the volatile 1992 presidential election compared to erratic polling trends.

• Specific State Race Projections

  • Maine: Initial fundamental analysis favored incumbent Republican Susan Collins by eight points due to her 2014 landslide victory; however, recent high-quality polling has shifted the forecast to favor Democratic challenger Sarah Gideon.
  • Alabama: The model projects a high probability of the seat switching from Democrat to Republican.
  • Flip Candidates: Arizona, Colorado, and North Carolina are identified as states with a strong probability of flipping toward the Democrats.

• Pandemic-Related Limitations

  • The forecasting model does not currently include explicit adjustments for the impact of the COVID-19 pandemic.
  • Analysts anticipate a record number of mail-in votes, introducing logistical complexity to the election process.
  • The model creator notes that the pandemic could produce "unmodelable effects" on voting behavior that would be difficult to predict without empirical evidence.
  • Predicting specific pandemic outcomes is currently deemed beyond the scope of statistical science due to a lack of hard data.