Or ever, really. What incentive is there for a party in opposition to ever confirm the opposing party's choice for the Supreme Court.

In past times there was a thing called compromise. Not very familiar anymore, I know. Instead of leaving a seat empty for the next ten years until the same party controls the Senate and the whitehouse (and not knowing ahead of time whether it'll be yours or not), the President can nominate a moderate which the opposition party Senate might confirm for the same reason. Better a moderate now than the other guy's candidate later.

But when you're looking at electoral math that says you're about to have even odds of taking the whitehouse and probably won't lose the Senate, that doesn't really apply.

That's probably the most insightful and (to my mind) intelligent assessment I've heard when asking that question. It eliminates any handwaving about "tradition" or "precedent" - and gets right down to the fundamental elements of power and control, and risk.