The dissent in the court case referenced by the question addresses this at some length. It takes care to distinguish two questions:
- Is a districting map fair to members of all parties?
- Does the map have the purpose and effect of diluting representation based on party affiliation?
The dissent agrees with the majority that the first question is not justiceable. But it identifies reasonable standards for adjudicating the second, based on standards applied by the district courts.
Some relevant excerpts from the dissent (citations omitted):
Page 15:
Over the past several years, federal courts across
the country—including, but not exclusively, in the decisions below—have largely converged on a standard for
adjudicating partisan gerrymandering claims (striking
down both Democratic and Republican districting plans in
the process). See also []. And that
standard does what the majority says is impossible. The
standard does not use any judge-made conception of electoral fairness—either proportional representation or any
other; instead, it takes as its baseline a State’s own criteria of fairness, apart from partisan gain. And by requiring
plaintiffs to make difficult showings relating to both purpose and effects, the standard invalidates the most extreme, but only the most extreme, partisan gerrymanders.
Page 16:
Start with the standard the lower courts used. ... both courts (like others around the country) used basically
the same three-part test to decide whether the plaintiffs
had made out a vote dilution claim. As many legal standards do, that test has three parts: (1) intent; (2) effects;
and (3) causation. First, the plaintiffs challenging a districting plan must prove that state officials’ “predominant
purpose” in drawing a district’s lines was to “entrench
[their party] in power” by diluting the votes of citizens
favoring its rival. [].
Second, the plaintiffs must establish that the lines drawn
in fact have the intended effect by “substantially” diluting
their votes. []. And third,
if the plaintiffs make those showings, the State must come
up with a legitimate, non-partisan justification to save its
map. See []. If you are a
lawyer, you know that this test looks utterly ordinary. It
is the sort of thing courts work with every day.
(In that last sentence, Kagan emphasizes that the standard need not be mathematically precise. The North Carolina gerrymander was created using sophisticated algorithms to precisely maximize one party's electoral gains at the expense of the other, so the notion of a counter-algorithm to set fair limits is intuitively appealing. But this is not how legal standards typically operate; phrases like "predominant purpose", "substantially", and "legitimate" are the norm, and exact quantitative formulas are the exception. As Kagan writes on page 27, "...courts all the time
make judgments about the substantiality of harm without
reducing them to particular percentages. If courts are no
longer competent to do so, they will have to relinquish,
well, substantial portions of their docket.")
Page 22-24:
Contrary to the majority’s
suggestion, the District Courts did not have to—and in
fact did not—choose among competing visions of electoral
fairness. That is because they did not try to compare the
State’s actual map to an “ideally fair” one (whether based
on proportional representation or some other criterion).
Instead, they looked at the difference between what the
State did and what the State would have done if politicians hadn’t been intent on partisan gain. Or put differently, the comparator (or baseline or touchstone) is the
result not of a judge’s philosophizing but of the State’s own characteristics and judgments. The effects evidence in
these cases accepted as a given the State’s physical geography (e.g., where does the Chesapeake run?) and political
geography (e.g., where do the Democrats live on top of
each other?). So the courts did not, in the majority’s
words, try to “counteract ‘natural’ gerrymandering caused,
for example, by the urban concentration of one party.”
[]. Still more, the courts’ analyses used the
State’s own criteria for electoral fairness—except for
naked partisan gain. Under their approach, in other
words, the State selected its own fairness baseline in the
form of its other districting criteria. All the courts did was
determine how far the State had gone off that track because of its politicians’ effort to entrench themselves in
office.
... In North Carolina, for example,
Democratic voters are highly concentrated in cities. That
fact was built into all the maps; it became part of the
baseline. See []. On top
of that, the maps took the State’s legal landscape as a
given. They incorporated the State’s districting priorities,
excluding partisanship. So in North Carolina, for example, all the maps adhered to the traditional criteria of
contiguity and compactness. See []. But the
comparator maps in another State would have incorporated different objectives—say, the emphasis Arizona
places on competitive districts or the requirement Iowa
imposes that counties remain whole. See []. The point is that
the assemblage of maps, reflecting the characteristics and judgments of the State itself, creates a neutral baseline
from which to assess whether partisanship has run amok.
Extreme outlier as to what? As to the other maps the
State could have produced given its unique political geography and its chosen districting criteria. Not as to the
maps a judge, with his own view of electoral fairness,
could have dreamed up.
Page 25:
According to the majority, “it does not make sense to use” a
State’s own (non-partisan) districting criteria as the baseline from which to measure partisan gerrymandering
because those criteria “will vary from State to State and
year to year.” []. But that is a virtue, not a
vice—a feature, not a bug. Using the criteria the State
itself has chosen at the relevant time prevents any judicial
predilections from affecting the analysis—exactly what the
majority claims it wants. At the same time, using those
criteria enables a court to measure just what it should: the
extent to which the pursuit of partisan advantage—by
these legislators at this moment—has distorted the State’s
districting decisions. Sure, different non-partisan criteria
could result, as the majority notes, in different partisan
distributions to serve as the baseline. []. But
that in itself raises no issue: Everyone agrees that state
officials using non-partisan criteria (e.g., must counties be
kept together? should districts be compact?) have wide
latitude in districting. The problem arises only when
legislators or mapmakers substantially deviate from the
baseline distribution by manipulating district lines for
partisan gain. So once again, the majority’s analysis
falters because it equates the demand to eliminate partisan gerrymandering with a demand for a single partisan
distribution—the one reflecting proportional representation. See []. But those two demands are
different, and only the former is at issue here.