During the 1990s, for instance, DoJ used its behind-the-scenes power to
push states to draw majority-minority districts. These efforts led to a dramatic
increase in the number of black and Latino legislators in Congress—and perhaps
to the election of more Republicans. Majority-minority districts tend to
concentrate Democratic votes, thus reducing the party's influence in other parts
of the state.
And let us never forget that concentrating votes on partisan lines, no matter how legal, effectively disenfranchises people who live on the wrong sides of the lines.