Such as today's ruling - Evenwel v Abbott - about how voting districts must adhere to the principle of "one person one vote". As described by Lyle Denniston over at SCOTUSBlog:
While virtually every argument used by the Ginsburg opinion in favor of basing representation on total population (because elected officials supposedly represent everybody and not just the voters) points toward a constitutional mandate, it turns out that the states actually are not bound by the Constitution to craft new election districts by starting with total population. The only thing settled constitutionally now is that the states also are not required to divide up districts by using the voting population to be assigned to each, making them equal. Should a state do it that way, the opinion seems to say, the Court will then face that issue.
Essentially the Court is telling us that they'll decide this matter again if a state DOES try to rewrite how they draw up districts by just the registered voters. Thing is, the way Ginsburg wrote this, the Court is dropping a huge hint that they'll side on having total population as the deciding factor.
Today's ruling makes it harder for states to, as the Washington Post's preliminary article about this case described it, redraw districts in more skewed gerrymanders that devastate major city populations: "A court ruling changing 'one person, one vote' to eligible voters (or some other metric) could flip many district sizes on their head. It would end up giving rural and more homogeneous districts more power than diverse urban areas, where there tend to be fewer eligible voters."
Say for example here in Florida: there may be 1.7 million people in Broward County/Ft. Lauderdale but only 1.1 million registered to vote. That's 600,000 people no longer counted towards carving out a district for the State Senate that has to draw out districts to fill 470,000 or so. Take away that 600,000 and Broward loses one full State Senate district to represent them. One less senator to represent a large county full of minority populations, young residents, and city residents relying on constant infrastructure repairs and upgrades. That would be a district that shifts outward to areas where there would be more registered voters - older and more rural - but fewer actual people.
The thing is, our elected officials are supposed to represent not just the ones who vote but everyone who resides within their scope of responsibility: everyone who lives in a State Representative's district, for example, even the ones who didn't vote for him/her. They're supposed to think of the children who are too young to vote, they're supposed to think of the poor who tend not to register to vote, they're supposed to think of everyone period when it comes time to pass legislation. In practice of course the Representatives heed their lobbyist buddies more, but the duty to the public remains...
Ginsburg spells that out specifically in her ruling (p.18):
As the Framers of the Constitution and the Fourteenth Amendment comprehended, representatives serve all residents, not just those eligible or registered to vote... By ensuring that each representative is subject to requests and suggestions from the same number of constituents, total population apportionment promotes equitable and effective representation...It's this passage that drops the hint to states that messing with "total population apportionment" isn't going to impress her or most of her fellow Justices.
Oddly enough, this decision had both sides losing in some way: the plaintiffs were denied the chance to have districts drawn up in a way that favored conservatives across many states, but the defendants - Texas and the Obama administration - were also denied having a confirmed Constitutional requirement to use total population as the redistricting metric.
Granted, the decision wasn't really unanimous - Alito wrote a concurrence that still questioned the final decision, and Thomas argued that the whole matter of "one person one vote" should still be left up to the states. But Ginsburg's ruling is as near a united decision that we'll ever see from a 5-4 bench split (well, 4-4 since the US Senate won't even speak to Garland right now, but I digress).
What matters here is that this ruling, relying on previous court decisions, is creating the framework to argue for stronger "one person one vote" requirements to voting rights overall. I'd like to think this can be the framework to argue for universal voter registration, to ensure that all possible residents of age (27th Amendment, sorry 5-year-olds...) have this right and the ability to employ it. Or to argue against gerrymanders that defy that "one person one vote" by pointing out how gerrymanders waste votes rather than ensure them.