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
| 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
| 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 |
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