What is this?

The MSC Simulator is an interactive tool for exploring how the ideological composition of the U.S. Supreme Court may evolve over the coming decades. It uses Monte Carlo simulation to model presidential elections, Senate control, justice retirements and deaths, and the ideology of future nominees under different institutional scenarios.

Credit

Based on Cameron & Kastellec, Making of the Supreme Court: The Politics of Appointments, 1930-2020 (Oxford University Press, 2023). The simulation methodology is described in Chapter 13, "The Courts That Politics Will Make." See also Conservatives may control the Supreme Court until the 2050s.

Precision

Browser results use 1,000 simulations. For the full R replication package, see the R replication package. The JavaScript engine is a faithful port of the R codebase, validated against R output to within sampling error.

Scenarios

Baseline

Standard model: elections, strategic retirement, and ideology drawn from calibrated Beta distributions. Transition probabilities are estimated from 1948–2021 historical data.

No Strategic Retirement

Removes the strategic retirement boost. Justices exit only through death or age-based basic retirement, regardless of whether the sitting president shares their ideological alignment.

No Divided-Government Confirmations

When the president and Senate are from different parties, vacancies remain unfilled until unified government is restored. Models a world where opposition Senates systematically block nominees.

GOP Senate Advantage

Senate control probabilities are shifted to favor Republicans (+0.10 to Dem switch rates, −0.10 to Rep switch rates), reflecting geographic clustering of Democratic voters. Vacancies persist until unified government.

Court Packing (+4)

Four new seats are added to the Court at the first unified government on or after 2026. New justices are appointed by the president in power and behave normally thereafter.

Tit-for-Tat Court Packing

Each party adds 2 seats when it gains unified government from the other party, leading to an ever-expanding court. A dedicated chart shows expected court size over time.

Term Limits (18 years)

Mandatory retirement after 18 years of service, with staggered onset beginning in 2026. Each seat transitions to the fixed-term system on a rolling schedule, ensuring regular turnover every two years once fully phased in.

Term Limits (9 years)

Mandatory 9-year terms, staggered by seniority with 1-year spacing. Each president appoints approximately 4 justices per term, producing rapid turnover.

2016 Counterfactual

Simulates a world where Hillary Clinton won the 2016 election and Merrick Garland was confirmed. Starts from a 5–4 liberal court in 2017 with transition probabilities recalibrated to pre-2016 data.

2026 Starting Court

The simulation begins with the current Court as of January 2026:

NSP = Newspaper-based Scores for Prediction. Positive values indicate conservative ideology; negative values indicate liberal ideology.