The main decision was a unanimous procedural action to bump the case back down to District Court. A concurring opinion from Kagan, Ginsburg, Breyer, and Sotomayor laid out a road may for the plantiffs to fix their case and come back and try again. I believe there is one more legal challenge not sufficiently pursued in this case.
The Procedural Rebuff
Chief Justice Roberts, et. al., primarily ruled that the plaintiffs had not sufficiently shown 'standing' for their case. They had to show that they had personally been harmed by the effects of gerrymandering. The plaintiff most cited, who must have done most of the talking during the original case, wasn't from a badly gerrymandered district. Even measuring gerrymandering is problematic and they don't like most of the measures.
The Court is aware of the mediocrity they have contributed to this situation. "Our previous attempts at an answer have left few clear landmarks for addressing the question." (p.8)
What they most wind up looking at is 'dilution of votes'. But, "To the extent the plaintiffs’ alleged harm is the dilution of their votes, that injury is district specific." (p.14) SCOTUS seems to want district by district, 'this one was packed', 'this one was cracked'. They disparage systemic, state-wide statistics, but want to focus on effects on concrete single voters.
The main decision claims that the fix could be small:
"Remedying the individual voter’s harm, therefore, does not necessarily require restructuring all of the State’s legislative districts. It requires revising only such districts as are necessary to reshape the voter’s district—so that the voter may be unpacked or uncracked, as the case may be." (p.16)
The concurring decision later suggests getting plaintiffs from every packed or cracked district, so that the result will wind up affecting pretty much all the districts.
On the "Efficiency Gap" measure:
"The difficulty for standing purposes is that these calculations are an average measure. They do not address the effect that a gerrymander has on the votes of particular citizens. Partisan-asymmetry metrics such as the efficiency gap measure something else entirely: the effect that a gerrymander has on the fortunes of political parties." (p.20)
7 of 9 members of the court felt that the case still had a chance though, so they bounced it back down to District Court. Thomas and Gorsuch wanted to just close the case and let the gerrymander stand.
The Concurring Hope
The Kagan, Ginsburg, Breyer, Sotomayor concurring opinion mostly calls out ways to make the anti-gerrymandering case better. They suggest that there's a good First Amendment claim to be made around freedom of association and the ability to effectively coordinate as a political party.
"... a plaintiff presenting such a theory would not need to show that her particular voting district was packed or cracked for standing purposes because that fact would bear no connection to her substantive claim."
"... everything about the litigation of that claim—from standing on down to remedy—would be statewide in nature." (p.2)
The main decision had a few notes, as SCOTUS frequently does, of "not our job" because they want lawmaking to be done by the legislatures and the congress, but this critique notes that,
"... only the courts can do anything to remedy the problem, because gerrymanders benefit those who control the political branches." (p.2)
They cite "Wesberry v. Sanders, 376 U. S. 1, 7 (1964)" for the "one-person, one-vote" principle and how fractionally diluting a vote is bad.
This to me sounds like an endorsement of the rising "ensemble" method of statistically challenging gerrymanders:
"In many partisan gerrymandering cases, that threshold showing will not be hard to make. Among other ways of proving packing or cracking, a plaintiff could produce an alternative map (or set of alternative maps)—comparably consistent with traditional districting principles—under which her vote would carry more weight." (p.3)
How to get a state wide effect for the state wide problem, more plaintiffs from all the packed and cracked districts:
"Assuming that is true, the plaintiffs should have a mass of packing and cracking proof, which they can now also present in district-by-district form to support their standing. In other words, a plaintiff residing in each affected district can show, through an alternative map or other evidence, that packing or cracking indeed occurred there." (p.6)
"... with enough plaintiffs joined together—attacking all the packed and cracked districts in a statewide gerrymander—those obligatory revisions could amount to a wholesale restructuring of the State’s districting plan." (p.7)
The main opinion claimed that the intent of the legislature didn't matter (documents were presented about deliberate gerrymandering) only the resulting injury to specific voters. Kagan, et.al., cite "Alabama Legislative Black Caucus v. Alabama" claiming that motive evidence counts and that statewide systemic evidence counts.
Getting back to the First Amendment free-association claim:
"The complaint in such a case is instead that the gerrymander has burdened the ability of like-minded people across the State to affiliate in a political party and carry out that organization’s activities and objects." (p.10)
"... nothing in the Court’s opinion prevents the plaintiffs on remand from pursuing an associational claim, or from satisfying the different standing requirement that theory would entail." (p.11)
The opinion closes with notes about how gerrymandering is bad and it hurts America and it has gotten a lot worse in the last two decades.
One More Thing
From the main opinion:
"The 99 members of the Assembly are chosen from single districts that must “consist of contiguous territory and be in as compact form as practicable.” §4. " (p.2)
What if the whole case is really much simpler? I can trivially show that the Wisconsin districts are not as compact as practicable. The Legislature of Wisconsin broke Wisconsin's laws. Just as in the recent Pennsylvania case, that's all you need. State court can throw out the gerrymandering.