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 YearDemocratsIndependentsRepublicansAll CongressMajority PartyVotes 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 YearDemocratsIndependentsRepublicansAll CongressMajority PartyVotes 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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