There is no such thing as withdrawing your support on principles. I wish I could get this point across to everyone who has supported Sanders, but thinks they can't vote for for Clinton.
Not voting isn't a message. No one knows (or cares) when you don't vote. Sure, each party would like your vote, but they couldn't give a rats ass if you don't vote. It isn't a blip on their radar.
The issues are still as important as they ever were. Trump or Clinton will be in the White House. Trump or Clinton will replace one, two, or three Supreme Court justices. Trump or Clinton will make Middle East policy. Trump or Clinton will make immigration policy. Like it or not, you *will* be part of determining which it is.
Not voting isn't supporting principles, it is completely and utterly abandoning them.