Imagine you are a political party. At present, you control the legislature and executive of your state government and you have the power to draw the maps that divide the state into Congressional and legislative districts. You want to ensure that you retain control of the legislature in future elections, even if the voting public shifts away from you. Can you do that with district maps?
Or you are a group of self-interested officials who can make their own individual lives easier by manipulating district maps. Remove the potential well-funded and popular competitor for your seat. Grab a few more likely voters and give away a neighborhood that you lost last time. Keep your biggest donor in your district. Can you do that with district maps?
Yes, that’s precisely what gerrymandering is. With publicly available information, one can draw maps that carefully allocate likely voters in your party and likely voters in the opposition party to districts in order to ensure that your party is overrepresented. The objective is to make the opposing party waste as many votes as possible and make your party waste as few as possible. We think of a vote as “wasted” if it goes to a losing candidate or if it goes to a winning candidate beyond the margin necessary to win. The two techniques for creating wasted votes are “cracking” and “packing.”
Cracking means to split your opponent’s votes across districts so that they don’t constitute a majority in any of them. When cracking, you want your opponent’s votes to be as large a minority as you can afford, although you want to leave a bit of a buffer to account for moderate shifts in voter preference. Cracking maximizes the number of wasted opposition votes that go to losers and minimizes the number of your wasted votes that go to winners.
Packing means to concentrate your opponent’s votes into a few districts where they constitute an overwhelming majority. You don’t mind if the opposition wins some seats, as long as they don’t win enough to threaten control. Packing maximizes the number of wasted opposition votes that go to winners and minimizes the number of your wasted votes that go to losers.
Preserving partisan power isn’t the only motivation for gerrymandering. Other forms of gerrymandering are employed to protect incumbents. For example, a district might be drawn to exclude a popular potential primary opponent. Before passage of the voting rights act in 1965, districts were gerrymandered to reduce the influence of minorities, particularly African-Americans. The practice persists today and since a Supreme Court decision in 2013, legal action has been required to remedy racial gerrymandering when it occurs.
We often think of gerrymandered districts as having strange-looking maps, and indeed, that is often the result. But districts with strange shapes are not necessarily gerrymandered and gerrymandered districts need not have strange shapes. Odd shapes are sometimes necessary to preserve the voting power of communities of interest or minority communities. And with modern mapping technology and data, it is possible to draw perfectly reasonably shaped districts with strong partisan biases.