Gerrymandering, the manipulation of the boundaries of an electoral district to predetermine the outcome based on party, race, incumbency, or other factors, has been happening for a long time. Traditionally, it was used to protect individual incumbents or to limit black and minority representation.
Typically, the state legislature redraws the boundaries of its state’s electoral districts with the new Census data available every ten years. With the 2020 Census coming up soon, there are efforts that some believe are meant to undercount hard-to-reach populations such as low-income households, minorities, and immigrants. (See my previous post for more detail.) If this occurs, it would mean that these residents will be under-represented when electoral districts are drawn, and, therefore, their voice and representation in state and federal legislative bodies would be diminished.
Gerrymandering has become more blatant, dramatic, and effective in the 21st century. It has been both fueled and exacerbated by partisanship and extremism in our state and national legislative bodies. It has been facilitated by increasingly sophisticated computer technology for mapping, analyzing, and tracking voters’ preferences and history. Historically, both Democrats and Republicans have engaged in gerrymandering.
Independent analyses find that in the redrawing of districts for the US House of Representatives following the decennial Censuses from 1970 to 2000, Democrats engaged in what’s called extreme partisan gerrymandering in one state after each of these four redistricting cycles. This occurred most dramatically in California in 1980. At its peak in the 1980s and 1990s, the best estimates are that through gerrymandering Democrats gained 3 – 5 seats in the House (out of 435 seats) above what would have otherwise been expected. After the 2010 Census, the Democrats did not engage in extreme partisan gerrymandering in any state. 
In redistricting after the 2000 and 2010 Censuses, independent analyses of the redrawing of districts for the US House find that Republicans engaged in extreme partisan gerrymandering in four states and seven states, respectively. The best estimates are that Republicans currently gain, through gerrymandering, between 15 and 20 seats in the House (out of 435 seats) above what would have otherwise been expected. A shift of 22 seats would change control from Republicans to Democrats.
For example, North Carolina is one of the states with extreme partisan gerrymandering of its Congressional districts. As a result, in 2012, Democrats got 51% of the votes for Congress statewide, but only won 4 of 13 seats in the House. In Pennsylvania, another state with extreme partisan gerrymandering, Democrats received just over half of the votes in 2012 but only 5 of 18 Congressional seats.  (This previous post has more information on the 2012 election results and on gerrymandering.)
Partisan gerrymandering has also dramatically affected thousands of seats in state legislatures. In Wisconsin, for example, in the 2012 election, Republicans received 49% of the statewide vote but got 60% of the seats in the Assembly of the state legislature. 
Extreme partisan gerrymandering has another, more insidious, effect. Nationwide, almost 100 of the 435 seats in the US House have been gerrymandered so only one of the two parties can win the seat. This means that the final election in November is meaningless for these seats. It also means that the voters of the party not in control of the district are effectively disenfranchised – their votes don’t matter (at least in terms of the election of their US Representative). Hence, tens of millions of voters effectively have no say in who is elected as their congressional representative.
In these congressional districts, gerrymandered to allow only one of the parties to win, the only election that matters is that party’s primary. Given the low voter participation in primary elections, a small number of voters, often ones with relatively extreme political views, determines who the US Representative will be. This is a significant contributing factor to the extreme partisanship and gridlock in Congress.
Extreme partisan gerrymandering insulates elected officials from all but a small handful of their constituents – those that vote for them in primary elections. Therefore, these congressional representatives do not need to worry about representing the interests of most of their constituents. When elected representatives redraw legislative districts after the Censuses and engage in gerrymandering, essentially the elected officials are picking their voters, rather than voters choosing their elected representatives.
This is clearly undermining democracy and the democratic principle of one person, one vote, i.e., that each voter has an equal voice in our democracy.
Partisan gerrymandering is accomplished by packing as many supporters of the opposition party into as few districts as possible. The opponents will win these seats overwhelmingly. Meanwhile, supporters of your party are spread more evenly across the other districts, so your party will comfortably win as many seats as possible. For example, in Pennsylvania in 2012, as the result of Republican gerrymandering, the Democrats won 5 congressional districts by an average margin of 76% to 24% (a 52 percentage point margin). The Republicans won 13 districts by an average of 59% to 41% (an 18 percentage point margin).  Clearly, if the Democratic voters had been spread out more evenly, the Democrats would have won more seats but by smaller margins. Overall, Democrats got about 350,000 votes and Republicans got about 250,000, but the Republicans won 13 of 18 seats. With fair districts, Democrats would have gotten 10 or 11 seats and Republicans 7 or 8 seats. So, extreme partisan gerrymandering produced a swing of 5 or 6 seats to the Republicans in Pennsylvania.
My next post will discuss what can be done to stop gerrymandering.
 Wang, S., & Remlinger, B., 9/25/17, “Slaying the partisan gerrymander,” The American Prospect (http://prospect.org/article/slaying-partisan-gerrymander)
 Li, M., 2/6/18, “What Pennsylvania’s landmark partisan gerrymandering ruling means,” Brennan Center for Justice (https://www.brennancenter.org/blog/what-pennsylvania-landmark-partisan-gerrymandering-ruling-means)
 Fried, C., 7/10/17, “Gerrymandering is unfair and unjust,” The Boston Globe
 Ballotpedia, retrieved from the Internet on 6/4/18, “United States House of Representatives elections in Pennsylvania, 2012” (https://ballotpedia.org/United_States_House_of_Representatives_elections_in_Pennsylvania,_2012)