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Party dominance in state legislatures

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Features of State Legislatures

Party dominance in state legislatures2012 Session TopicsStanding committees analysis for 2011-2012 sessionLength of terms of state representativesHow vacancies are filled in state legislaturesStates with a full-time legislatureState legislative chambers that use multi-member districtsState legislatures with term limitsComparison of state legislative salariesWhen state legislators assume office after a general electionPopulation represented by state legislatorsState constitutional articles governing state legislaturesState legislative sessionsResign-to-run laws
This page explores the relative dominance of state legislatures by one political party. To explore the levels of dominance across different states, a metric rating each state on a scale of 0 to 1 is used to provide a direct comparison between proportional levels of dominance in each state.

Measuring Party Dominance

To calculate a measure for the seats in state legislatures controlled by one party, the proportion of each chamber controlled by the Republican Party provides a metric for Republican dominance (a high proportion), Democratic dominance (a low proportion), or no dominance by either party in the legislature (near 0.50). These two percentages (one for each chamber) are then added together to provide a measure of the degree to which Republicans control the state legislature (with 2 being complete control and 0 indicating control of no GOP seats). By subtracting 1 and taking the absolute value of the resulting number, the state is assigned a rating between 0 and 1 where a higher number indicates greater dominance by the majority party in the state legislature.


Consider a state has a state house with 100 representatives and a state senate with 50 members. If the Republicans control 90 seats in the state house and 40 seats in the state senate, with Democrats controlling the remaining balance of the seats, then the calculation would be 0.90 (90 percent of state house seats) plus 0.80 (80 percent of state senate seats), minus 1. The resulting 0.7 shows a high degree of one-party dominance.

|((40/50)+(90/100))-1| = |(0.80 + 0.90)-1| = |0.70| = 0.70

Consider the same state with Democrats in control, with 30 seats in the senate and 60 seats in the house, with Republicans taking the balance of the seats. The absolute value calculation allows the dominance of either party to be measured without considering which party is the one that is dominant. A result of 0.20 shows some, but not overwhelming, party dominance.

|((20/50)+(40/100))-1| = |(0.40 + 0.40)-1| = |-0.20| = 0.20

If the same state had 25 Republicans and 25 Democrats in the senate and 50 Republicans and 50 Democrats in the house, then the calculation should be 0.50 plus 0.50 minus 1, with the absolute value of that being 0, showing no party dominance.

|((25/50)+(50/100))-1| = |(0.50 + 0.50)-1| = |0| = 0

Consider the example state used above where Democrats control 30 of the state senate seats (with Republicans given the balance) and Republicans control 90 of the state house seats.

|((20/50)+(90/100))-1| = |(0.40 + 0.90)-1| = |0.30| = 0.30

Party Dominance as of August 27, 2012
Party dominance map.png
This map shows partisan dominance as of August 27, 2012. The 5 most dominant
and least dominant states are highlighted by party.


This measure does not indicate which party dominates a legislature. This measure can also give a sense of lower levels of dominance if misinterpreted. For example, if one party controls 70 percent of the seats in a senate and 70 percent of the seats in a house, the state would still only have a dominance rating of 0.40, which is lower than one might expect with such a high level of dominance by one party. Finally, this measure also assumes a two-party system, and does not provide a metric for sizable third parties.

Party Dominance Data

See also