NOTE: I HAVE UPDATED “U.S. SUPREME COURT: LINES OF SUCCESSION AND IDEOLOGICAL ALIGNMENT” TO REFLECT THE CONFIRMATION OF BRETT MICHAEL KAVANAUGH AS AN ASSOCIATE JUSTICE OF THE U.S. SUPREME COURT.

Justice Elena Kagan — voicing the fears of leftists — says this about the replacement of Anthony Kennedy by Brett Kavanaugh:

I think it’s been an extremely important thing for the court that in the last, really 30 years, starting with Justice O’Connor and continuing with Justice Kennedy, there has been a person who people — found the center where people couldn’t predict in that sort of way. And that’s enabled the court to look as though it was not owned by one side or another, and was indeed impartial and neutral and fair. And it’s not so clear that — I think, going forward, that sort of middle position — you know, it’s not so clear whether we’ll have it.

All of us need to be aware of that — every single one of us — and to realize how precious the court’s legitimacy is….

It’s an incredibly important thing for the court to guard is this reputation of being impartial, being neutral and not being simply an extension of a terribly polarizing process.

So the job of the Supreme Court isn’t to uphold the Constitution, but to find a middle ground between constitutional and anti-constitutional views.

I am hopeful — but not yet certain — that the addition of Justice Kavanaugh to the Court’s lineup will end the three decades of uncertainty praised by Kagan, and that it will put the Court back in the business of firmly upholding the Constitution. A business that it began to abandon in earnest during the New Deal.

Drawing on statistics kept at SCOTUSblog, I have constructed an index of defection (D) for each justice, for the 2005-2017 terms:

D = percentage disagreement (in non-unanimous cases) with members of own wing/percentage disagreement with members of opposite wing.

The wings are the “conservative” wing (Gorsuch, Alito, Thomas, Scalia, Roberts, and Kennedy) and the “liberal” wing (Breyer, Ginsburg, Kagan, Sotomayor, Souter, and Stevens).

The lower the index, the more prone is a justice to vote with the other members of his or her wing; the higher the index, the more prone is a justice to vote with members of the opposing wing. Here’s a graph of the indices, by term:

Kennedy’s long-standing proneness to defect more often than his colleagues grew markedly in the 2014-2015 terms and receded a bit in the 2016 term. His turnaround in the 2017 term restored him to the Court’s “conservative” wing.

Roberts slipped a bit in the 2017 term but was still more in step with the “conservative” wing than he had been in the 2014-2015 terms.

Gorsuch started out strongly in his abbreviated 2016 term (he joined the Court in April 2017). His slippage in the 2017 term may have been due to the mix of cases at stake.

What’s most striking about the preceding graphs, other than Kennedy’s marked departure from the “conservative” wing after the 2010 term, is the increasing coherence (ideological, not logical) of the “liberal” wing. This graph captures the difference between the wings:

The record of the past six terms (2012-2017) is clear. The “liberals” stick together much more often than the “conservatives”. Perhaps that will change with Justice Kavanough on the Court.