Hillary Clinton just got one of the most noteworthy signals of approval any politician could ask for — a criticism of her likely Republican opponent from a sitting justice of the U.S. Supreme Court.

In an interview with the New York Times, Ruth Bader Ginsburg said, “I can’t imagine what this place would be — I can’t imagine what this country would be — with Donald Trump as our president.” She recalled something her late husband used to say: “Maybe it’s time for us to move to New Zealand.”

The issue here isn’t Trump’s fitness for the presidency — we’re beyond dubious. Nor is it Ginsburg’s freedom to think whatever she wishes of him.

But to say her public comments are unusual is like saying dancing cows are scarce. Supreme Court justices don’t — at least until now — take public stands on elections. One reason is that they are barred from doing so by the federal code of judicial conduct, which states that as a general rule, judges shall not “publicly endorse or publicly oppose another candidate for public office.” They also aren’t allowed to make speeches on behalf of political organizations or give money to candidates.

The reasons are clear and sensible. Judges who sit on the federal bench are protected from political pressures by the life tenure provided in the Constitution. Courts are often asked to rule on matters of public controversy, and the litigants on either side are entitled to expect that the presiding judges will evaluate their arguments fairly.

Nowhere is that impartiality more important than in the highest court in the land, which has the final word on a host of grave questions. For justices to descend into partisan election campaigns would undermine public faith in their willingness to assess each case strictly on its legal merits.

Yet here was Ginsburg plainly indicating how she will vote in this election. “I am not aware of any justice ever expressing views on the merits or demerits of a presidential candidate in the midst of the campaign,” Ed Whelan, president of the Ethics and Public Policy Center, told the Washington Post. George Washington University law Prof. Jonathan Turley told us her comment “showed manifestly bad judgment and undermined the integrity of the court.”

She is not the first justice to play fast and loose with these obligations. But Ginsburg went further by publicly stating her opinion of a candidate during a campaign. If Trump is elected, Ginsburg may have to rule on the legality of policies he pursues. Can anyone assume she’ll put aside her disdain for him and give them a fair hearing? If President Trump were to lose a case, wouldn’t he have plausible grounds to claim, as he often does, “The system is rigged”?

The Supreme Court commands respect partly because it stands outside of petty politics. When a member of the court strides into that muck, she hurts the cause of justice.