As a practical matter, the ruling mostly helped Democrats.

Until this decision, the court had never resolved whether voting districts should contain the same number of people, or the same number of eligible voters. Counting all people amplifies the voting power of places that have large numbers of residents who cannot vote legally — including immigrants who are here legally but are not citizens, illegal immigrants, children and prisoners. Those places tend to be urban and to vote Democratic.

Had the justices required that only eligible voters could be counted, the ruling would have shifted political power from cities to rural areas, a move that would have benefited Republicans.

The court did not decide whether other ways of counting were permissible. “We need not and do not resolve,” Justice Ruth Bader Ginsburg wrote for six justices, whether “states may draw districts to equalize voter-eligible population rather than total population.”