The U.S. relationship with Israel, and what to do about Iran's nuclear program, represents one of the starkest contrasts between Obama and Romney. Romney has not explicitly threatened a U.S. military strike on Iran if he is elected. But he has suggested he would take a tougher stance than Obama.

"If we re-elect Barack Obama, Iran will have a nuclear weapon. And if you elect Mitt Romney, Iran will not have a nuclear weapon," Romney said last year at a GOP primary debate focused on foreign policy.

Obama rejects the criticism that he has been weak in dealing with Iran, and his aides point to what they call unprecedented U.S.-Israeli security cooperation