I’ll join Orin in some speculation. For the dominance of Catholics appointed by Republicans, I think Alkali, commenting on Orin’s thread, identifies part of the reason:
A Republican president can’t win by nominating a Protestant: if the nominee is a conservative evangelical, that could be controversial with moderate voters; if the nominee is not a conservative evangelical, that would create problems with the conservative base. That dynamic strongly favors nominating a Catholic or Jewish justice. The fact that no current Republican-nominated justice is Jewish is very likely just an artifact of chance — had Douglas Ginsburg’s nomination succeeded, it wouldn’t be the case.
But we can’t neglect the issue of Harvard/Yale laws schools dominance of the nominee pool. (There were many fair criticisms of Harriet Miers, but the fact that she didn’t attend Harvard or Yale like her potential USSC colleagues was not one of them!) Both schools have a lot more conservative Catholics than conservative Protestants, and of course Catholics are much more likely to be conservative Republicans, and especially to have anti-abortion sympathies, than are Jews. If I’m remembering correctly, a Catholic was the president of the Federalist Society all three of the years I attended Yale.
As for Jews and the Democrats, Yale was at least one-third Jewish when I attended, and I think has been so for a long time (less true at Harvard, but Harvard’s Jewish quotas were long gone by Kagan’s time). (A proposed song at the annual satirical show, eventually and wisely thought better of, was “Two Jews for every Goy” sung to the tune of “Two Girls for every Boy.”) The Jews, overall, were probably not much more liberal than the class as a whole, but let’s say that 40% of the graduating liberal Democrats were Jews. The percentage would be slightly higher if we limit ourselves to graduating liberal Democratic women.
The percentage would likely be even higher if we limited ourselves to “liberal Democratic women who graduated from Yale or Harvard and chose to pursue careers at the upper echelons of the legal profession in the Boston-Washington corridor,” which is where all the Justices have come from since Clarence Thomas [the Boston-Washington corridor, that is]. With those background statistics in mind, and given Obama and Clinton’s eagerness to find women for the Court, 75% of the Democratic appointees on the Court being Jewish doesn’t seem outside the range of normal statistical chance.