He won’t defend Israel. Mormon teaching is that THEY are the true Israel (replacement theology). They are also very friendly to Muslims.
I would vote for a Jew long before I would vote for a Mormon. At least they believe in the same God of Israel.
If Romney won’t support Israel then this will doubtless come out in the wash, through the primaries. America’s evangelical side strongly supports Israel. It will be a rejection of a consequence of Mitt being a faithful LDS (if he is — seems to me that he isn’t strongly dedicated even to boasted Mormon ideals).