The practical effect of the court’s decision is likely to be seen in declining racial diversity among elected leaders, said Pamela S. Karlan, a law professor at Stanford University and a former Justice Department official in Democratic administrations, as Republican lawmakers in the South target districts held by Black Democrats insisting they are doing so merely for partisan advantage.

“Now that they’ve decided that there’s just no problem at all in having mono-racial political delegations from the South, it’s likely that’s what we will start seeing,” Ms. Karlan said.