This resolves positive if a future Republican president chooses a replacement for Sonia Sotomayor. For example, if Trump wins the coming election, Sotomayor leaves the court in 2026, and Trump chooses her replacement, it will resolve positive.

It resolves negative if a Democratic president chooses Sotomayor's replacement. It will also resolve negative if something radical happens to the structure of the Supreme Court or partisan politics that makes this question incoherent.

This market only resolves once a nominee has been confirmed. If a nominee isn't confirmed, for example because the President and Senate can't agree, it will remain open until the President and Senate do agree on someone, and the party of the President whose nominee was eventually confirmed will be used to resolve this market. If Sotomayor is not replaced by 2060, this resolves N/A, unless at that point Manifold won't let me resolve markets N/A, in which case it resolves no.

For context and the reason I'm interested in this, see 

https://www.natesilver.net/p/sonia-sotomayors-retirement-is-a