It may be wise, as you suggest, to restrict attention to 5-4 decisions, since that is when the vote “counts.” I did my own study, and found that Scalia and Thomas were on the same side in 102 out of 113 five-four decisions, more than any other pair.
From Wikipedia, I reconstructed a database of 580 Supreme Court decisions for the years 2001-2009. Of these, 113 decisions were made by a 5-4 vote. I confined my analysis to those 113 cases.
I found that the 4 “liberals” (Stevens, Ginsburg, Breyer and either Souter or Sotomayor) voted the same way on 83 of the cases, split 3-1 on 18 of the cases, and split 2-2 on 12 of the cases.
Of the 83 five-four decisions where “liberals” were united, they lost 52 and won the remainder with the help of Kennedy (18), O’Connor (10), Scalia (2), Thomas (1).
Of the 18 five-four decisions where “liberals” were split 3-1, they lost 11 despite help from O’Connor (5), Kennedy (2), Scalia (2), Thomas (1), Rehnquist (1), and won 7.
By the way, when groups of 2 judges are considered instead of groups of 4, no pair of “liberals” outperformed Scalia and Thomas for correlation: this pair were on the same side in 102 out of the 113 five-four decisions.
Here are the detailed votes in the 113 five-four decisions:
The 12 vote columns correspond to
- Rehnquist
- Roberts
- Stevens
- O’Connor
- Scalia
- Kennedy
- Souter
- Thomas
- Ginsburg
- Breyer
- Alito
- Soto
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