I’m reading/skimming the decision on partisan gerrymandering, and I can’t say that I necessarily disagree with this point from the majority’s opinion (they ruled that partisan gerrymandering is a “political question” outside the Court’s purview):

Even the most sophisticated districting maps cannot reliably account for some of the reasons voters prefer one candidate over another, or why their references may change. Voters elect individual candidates in individual districts, and their selections depend on the issues that matter to them, the quality of the candidates, the tone of the candidates’ campaigns, the performance of an incumbent, national events or local issues that drive voter turnout, and other considerations. Many voters split their tickets. Others never register with a political party, and vote for candidates from both major parties at different points during their lifetimes. For all of those reasons, asking judges to predict how a particular districting map will perform in future elections risks basing constitutional holdings on unstable ground outside judicial expertise.

It is hard to see what the District Court’s third prong— providing the defendant an opportunity to show that the discriminatory effects were due to a “legitimate redistricting objective”—adds to the inquiry. 318 F. Supp. 3d, at 861. The first prong already requires the plaintiff to prove that partisan advantage predominates. Asking whether a legitimate purpose other than partisanship was the motivation for a particular districting map just restates the question.

That said, as Kagan points out in her dissent, there are existing tests that are pretty standard tests that lawyers and courts use all the time to deal with claims of vote dilution.

Lastly, I’ll say this: I don’t necessarily blame the majority for punting; nor do I disagree with the dissent that there are viable tests that can be used. However, gerrymandering can often times be an “eye of the beholder” situation (or to use Justice Potter’s famous line “I know it when I see it”). I think districts should generally be compact and where possible not split counties/cities/neighborhoods. But even with that basic formulation, it’s possible to create maps that favor one party over the other.

Post-script: There is, of course, a political answer to these issues. Register more voters, turn them out at elections, invest in races with good candidates, win seats “you shouldn’t have been able to win”, win the governor’s race in cycles where redistricting will happen