I think Obama will have pretty much free reign to nominate a replacement for her if she leaves. She's liberal enough that you can't push the court too hard to the left.
The only credible names he probably couldn't get confirmed was her replacement are Harold Koh (would rule based on international law rather than US law), Debo Adegbile (ties to Mumia), and Eric Holder (many problems, plus likely too old).
The Republicans have bigger fights to pick than a tit-for-tat replacement.
Now if Kennedy was to retire, Obama likely couldn't get any liberal-leaning judge but Merrick Garland through as a replacement.
Literally nobody has brought up those names, and why would they?
And I don't get that bit about Koh. Even if it was the case, it'd be good. We need more European style law.
I definitely remember Garland's name coming up a lot when Souter retired, but Obama chose Sotomayor instead.
I know this wouldn't happen with a Democrat in the White House, but I'd like to second whoever it was that suggested Brian Sandoval. I think he would do very well as an Associate Justice.
