When the question is whether the Court will police extreme partisan gerrymanders, certain Justices--most notably Chief Justice John Roberts--voice concerns about threats to the Court's public legitimacy if it is seen to be intervening in partisan politics and squandering its diffuse public support, even though policing extreme partisan gerrymanders would not systematically favor one political party over the other.
When the question is whether the Court will permit the Trump Administration to add a citizenship question to the census that will obviously benefit the Republican Party and harm the Democratic Party and that obviously has the purpose of causing that effect, those same Justices--again, most notably the Chief Justice--do not voice concerns about threats to the Court's public legitimacy if it is seen to be intervening in partisan politics and squandering its diffuse public support, even though permitting the census question will systematically favor one political party over the other.
Instead, at least judging from the recent oral argument in the census case, the Court seems poised to defer to the Trump administration by a vote of 5 Republican appointees to 4 Democratic appointees. The Court appears poised to do so based on the so-obvious-it's-insulting pretext that the Administration's reasons for adding the citizenship question are to enable it to better enforce the Voting Rights Act. No matter that nonpartisan experts conclude that adding the citizenship question will not enhance VRA enforcement; that the Trump Administration has shown little interest in VRA enforcement; and that the Court's conservatives are otherwise increasingly hostile to the longstanding doctrine that courts should defer to reasonable judgments by administrative agencies.
It is the cases in which the future fortunes of the two main political parties are at issue that the Justices should be most scrupulous about appearing--and actually being--nonpartisan. It is in such cases that they should avoid leaving every impression of really, really wanting to come out in favor of the interests of the party that appointed them and asking themselves only how they can best negotiate the legal materials and arguments in order to find the path of least resistance. It is in such cases that we all learn whether their robes only appear black but are really red or blue--whether they are simply judges or whether they are really Trump or Obama judges.
Talk is cheap. See, e.g., Bush v. Gore (2000).