There was another alternative than VETOing the UN resolution:
Abstain.
This would also be seen as weakness and an anti-Israeli action by the electorate and would be one of the top 3 talking points for any Republican candidate.
But that would hint at Obama being a liberal. Which he isn't.
I've always thought of Obama as being a pragmatic liberal that moderates his stances for political reasons (i.e. with the 2012 elections in mind). One of the oddities of American politics is that a second term president is a lame duck president the very first day of the term.