The Political Pendulum: Party Control and Strength in Congress

Political power in the United States tends to shift back and forth between two parties, creating a political “pendulum.” Some shifts take longer or are more decisive than others as each party vies for control over the U.S. House of Representatives and Senate.

But party control is more nuanced than the number of seats that are blue or red. The strength of those seats has implications: Were most seats won through narrow victories, indicating weaker support than their control suggests? Did one party easily gain control of Congress, defeating some incumbents along the way?

datascience@berkeley created this website to show the timing of the pendulum and its speed and strength over time. Below is the split in party control of each Congress, as well as the strength of that split — by how much did each congressperson win their last election?

In the tables below are the median win percentages for each party in the House of Representatives and the Senate. A percentage higher than 50 suggests that the median candidate won their seat by a higher amount of votes. A percentage closer to 50 suggests that race was much closer.

Median Win Percentage – House of Representatives

Median Win Percentage – House of Representatives
Election Year Democrats Independents Republicans All Congress Majority Party Votes over Majority
1980 67.7% 37.8% 66.9% 67.2% Democrats +25
1982 67.2% 0.0% 62.2% 65.1% Democrats +52
1984 66.9% 0.0% 68.6% 67.5% Democrats +36
1986 72.2% 0.0% 67.0% 70.2% Democrats +41
1988 70.8% 0.0% 69.5% 70.3% Democrats +43
1990 66.0% 57.5% 63.7% 65.2% Democrats +50
1992 62.6% 57.8% 60.8% 61.4% Democrats +41
1994 62.3% 49.9% 65.7% 64.0% Republicans +13
1996 64.8% 52.9% 61.9% 63.0% Republicans +9
1998 68.6% 63.4% 66.0% 67.2% Republicans +6
2000 68.7% 68.3% 64.9% 67.0% Republicans +4
2002 67.6% 64.3% 67.8% 67.6% Republicans +12
2004 68.7% 67.5% 65.0% 66.8% Republicans +14
2006 69.5% 0.0% 60.3% 64.5% Democrats +19
2008 68.9% 0.0% 61.1% 65.3% Democrats +40
2010 61.1% 0.0% 63.8% 63.0% Republicans +25
2012 65.7% 0.0% 60.9% 62.5% Republicans +17
2014 64.8% 0.0% 64.6% 64.6% Republicans +30
2016 67.9% 0.0% 62.7% 64.1% Republicans +24

Median Win Percentage – Senate

Median Win Percentage – Senate
Election Year Democrats Independents Republicans All Congress Majority Party Votes over Majority
1980 59.4% 57.2% 53.8% 56.0% Republicans +3
1982 60.8% 0.0% 53.5% 56.0% Republicans +4
1984 60.8% 0.0% 52.4% 55.8% Republicans +3
1986 60.8% 0.0% 55.2% 56.9% Democrats +5
1988 59.5% 0.0% 56.4% 58.1% Democrats +5
1990 60.4% 0.0% 60.5% 60.4% Democrats +6
1992 59.2% 0.0% 55.9% 58.6% Democrats +7
1994 58.0% 0.0% 56.9% 57.7% Republicans +3
1996 54.5% 0.0% 55.4% 55.1% Republicans +5
1998 55.6% 0.0% 56.7% 56.5% Republicans +5
2000 55.7% 0.0% 58.5% 56.8% Democrats +1
2002 59.4% 0.0% 63.2% 61.1% Republicans +1
2004 60.6% 0.0% 5.5% 59.9% Republicans +5
2006 60.3% 57.6% 58.2% 59.4% Democrats +2
2008 61.4% 57.6% 57.9% 61.2% Democrats +8
2010 59.4% 57.6% 58.6% 58.9% Democrats +3
2012 56.9% 62.0% 57.8% 57.7% Democrats +5
2014 55.4% 62.0% 57.8% 57.8% Republicans +4
2016 55.7% 62.0% 57.9% 56.6% Republicans +1

Created by datascience@berkeley, the online Master of Information and Data Science from UC Berkeley


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