Obama knows if he signs it, it will stand, even a Republican gets the WH. Because once a treaty is signed, its rare for a following president to "unsign" it, even if they disagree with it (and even unsigning may not actually get you out) and the Senate hasn't ratified. Because you know international relations are more important then your own citizens.
Few people seem to know that a signed treaty, even if its not ratified by the Senate is not dead forever. IT CAN BE RATIFIED by the Senate at any time later, even many years later. In fact they never expire or have a sunset date. You may remember the high seas treaty that reared its ugly head just before the 2012 election. And this one has never been signed.
http://www.foxnews.com/politics/2012/07/16/sea-treaty-all-but-dead-34-gop-senators-oppose.htmlThe high seas treaty has been around since the late 1950's. It was in the news in the 1980's but went nowhere because Reagan opposed it. It was updated during the 1990's and Kerry was pushing it.
So even if its dead on arrival now, doesn't mean it can't be reborn later.