Recognition of a Palestinian state by upgrading it's status to observer state would give Palestinians more leverage against Israel. Israell has done nothing to clarify its borders and obstructs the process. The move is positive for the Palestinians as it will give them and Israel a more legally clearer definition of the state and Israel will have no choice but to negotiate instead of obstruct. America is simply trying to keep the status quo of no advance in the process.
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from the article: Palestinians: UN recognition would foster peace with Israel