State Legislative Bipartisanship Scores

Mackenzie R. Dobson · University of Tennessee, Knoxville · State Politics & Policy Quarterly, 2026

The State Legislative Bipartisanship Scores (SLBS) provide original measures of bipartisan collaboration for 10,817 legislators serving in 43 U.S. state legislatures between 2009 and 2018. Comprising 27,129 legislator-term observations, the dataset captures two complementary dimensions of bipartisan engagement: the extent to which legislators support bills introduced by members of the opposite party and the extent to which they attract support from across party lines on their own legislation. Drawing on millions of bill sponsorship and cosponsorship relationships, the measures provide a comparable framework for studying bipartisan behavior across states, chambers, parties, and legislative contexts.

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27,129
Legislator-term obs.
43
State legislatures
86
Chambers
2009–2018
Time series

Measures

The SLBS dataset includes two measures of bipartisan collaboration calculated for each legislator during each legislative term. Together, they capture both sides of bipartisan engagement: the willingness to support legislation introduced by members of the opposite party and the ability to attract support from members of the opposite party on one's own legislation.

Bipartisan cosponsorships offered
proportion_direct_bipartisan_bills

Measures the share of a legislator's cosponsorship activity directed toward bills introduced by members of the opposite party. Higher values indicate a greater willingness to extend support across party lines.

= Bill sponsor = Cosponsor (legislator i) Legislator i cosponsors 5 bills this term BILL 1 out-party BILL 2 out-party BILL 3 same party BILL 4 same party BILL 5 same party out-party bills cosponsored ÷ total cosponsored = 2 ÷ 5 = 0.40 (or 40%) Legislator i's offered score = 0.40
Bipartisan cosponsors attracted
average_prop_bill_bipartisan

Measures the average share of opposite-party cosponsors on the bills a legislator sponsors. Higher values indicate a greater ability to build cross-party support for one's legislative agenda.

= Sponsor (legislator i) = Out-party cosponsor = Same-party cosponsor Legislator i sponsors 2 bills this term BILL 1 (sponsored by i) 4 out-party / 4 total = 1.00 BILL 2 (sponsored by i) 0 out-party / 4 total = 0.00 average out-party share across all bills = (1.00 + 0.00) ÷ 2 = 0.50 (or 50%) Legislator i's attracted score = 0.50

Bipartisanship and chamber competition

This map explores how bipartisan collaboration varies across different legislative environments. States are classified according to the level of majority-party security within each chamber during the time series. Majority security is measured using partisan seat share difference, defined as the absolute difference between the proportion of seats held by the majority and minority parties. For example, a chamber where one party holds 60 percent of the seats and the other holds 40 percent would have a partisan seat share difference of 0.20.

Chambers are classified as secure when the partisan seat share difference is 0.33 or greater, meaning the majority party holds at least two-thirds of the seats in the chamber. Chambers below this threshold are classified as insecure and represent more competitive partisan environments.

Use the House and Senate toggle to explore how bipartisan collaboration differs across varying levels of majority-party security. Clicking a state reveals chamber-specific information on both bipartisan cosponsorship behavior and partisan competition.

Secure majority (modal seat share diff. ≥ 0.33)
Insecure majority (modal seat share diff. < 0.33)
Not in dataset

Modal security classification is the most common security designation across all available legislative terms for each chamber within the 2009–2018 time series. A chamber is classified as secure when partisan seat share difference is 0.33 or greater in the majority of its observed terms.

Chamber-level summary table

This table summarizes bipartisan collaboration and chamber competition across state legislative chambers included in the time series. Values represent averages across all available legislative terms for each chamber.

Majority offered and Minority offered report the average share of cosponsorships directed toward bills sponsored by members of the opposite party among legislators serving in the majority and minority parties, respectively. Average seat share difference measures the degree of partisan imbalance in the chamber and is calculated as the absolute difference between the seat shares of the two parties.

Click any row to view the corresponding chamber on the map and explore its bipartisan collaboration and competition profile.

State Chamber Avg. seat share diff. Majority mean offered Minority mean offered Gap (min − maj)

All values are pooled averages across all available legislative terms for each state-chamber within the 2009–2018 time series. Term coverage varies by state.

Citation

Dobson, Mackenzie R. 2026. "Selective Reciprocity in Bipartisan Collaboration: How Majority Security Shapes Legislative Success." State Politics & Policy Quarterly 26(2): 139–164. doi:10.1017/spq.2026.10021

Dobson, Mackenzie R. 2026. Replication data for: Selective Reciprocity in Bipartisan Collaboration: How Majority Security Shapes Legislative Success. V1. Harvard Dataverse. https://doi.org/10.7910/DVN/W7OXER

All values are averages pooled across all available legislative terms for this chamber within the 2009–2018 time series. Modal security reflects the most common classification across observed terms.