[Edit 8/4/2019: The recent U.S. Supreme Court case of Rucho vs. Common Cause has rendered this post partially inaccurate. See explanation at the end.]

In talking with friends, I’ve found there is some confusion over why redistricting is important, First of all, what is redistricting? It’s when a state government divides up the state into voting districts for election purposes, such as for Congressperson or state assembly. It could also be the same action by a city or county government for local seats like councilman. The U,S. Constitution as interpreted by the courts requires that each person’s vote be of equal value, sometimes called the “one man, one vote” rule. In practical terms this means that districts must be drawn to have roughly equal number of voters in each. This can be done fairly or unfairly. It is usually done by a committee appointed by a governor and thus dominated by the governor’s party, although methods vary in different states. Redistricting committees usually end up trying to protect their party’s power, or at least protect specific incumbents.The Constitution requires a census every ten years followed by redistricting to ensure that Congress reflects the current population distribution.

Here are three examples to illustrate why it’s important. Assume the state in the drawing has five congressional seats and two parties we’ll call orange and green represented by the 20 icons. The voters are 60% green (12 icons) and 40% orange (8 icons) distributed as shown.

One would expect that two of the five seats (40%) would go to the orange party and three (60%) to the green if lines were drawn in a neutral, unbiased manner. Courts, by the way, have also ruled that lines must be drawn logically from a geographic viewpoint, too. The next image illustrates what would probably be considered fair.

Based on where majorities are located, orange would get two seats and green three, and the districts appear to be logical geographically, too. However, if green is in control of redistricting it might try to draw the lines as follows:

See how that almost certainly gives Green four of the five seats since they have a 3-to-1 voter ratio in four. Notice also that the districts have odd shapes. This is where the term gerrymandering comes from – districts drawn this way are said to resemble salamanders. Courts often strike these down and have been known to appoint their own committees or special masters to redraw the lines more fairly.

Now consider the following district lines.

There are two safe districts for Green in the middle, but the other three districts are split 50-50. They could go either way. The district lines appear to be rational from a geographic standpoint. A court would probably consider this fair even though it might result in Green representatives in all five districts or three being Orange. However, suppose the two purplish districts are currently represented by popular Orange incumbents who consider their seats safe. The Orange party, if they’re lucky enough to be in control, might like this one better than the first (fair) one above, assuming the incumbents aren’t worried about the 50-50 split in their districts. This way they have a 50-50 chance at picking up a third Orange seat. In real life it is quite possible, and has often happened, that a minority party can retain, even guarantee, control in its state.

A friend recently seemed worried that redistricting could affect the U.S. Senate. No, it can’t. Why not? Because there are no U.S. Senate districts. The entire state is the one and only district for any Senate race. There are simply no lines to draw. You can’t redraw state boundaries. Every voter in the state can vote on every U.S. Senate race in their state. State senates, however, like state assemblies or legislatures, have districts, so they are subject to being gerrymandered.

Edit: The court in the  Rucho case ruled in effect that gerrymandering on the basis of party is okay. It is only gerrymandering on the basis of race that is outlawed by the constitution according to current Supreme Court law. If you substitute black and white voters for the green and gold, then the example and illustrations are accurate.