Israel in not willing to currently give up more than 60% of W. Bank or deal with refugees. The PA is boxed in with no good options. The US would be in bad position to either veto out right or try to delay it for 2yrs to get peace talks one last chance. A veto would cause US alot of problems with troops in 2 Muslim countries and need to control Iran's nukes.
U.S. Democratic Sen. Bill Nelson announces support for Iran nuclear deal (Haaretz)
from the article: Netanyahu seeks to foil Palestinian declaration of statehood, again