Pressuring Israel to form a "Palestinian" state is only one way which the president can have a negative impact on Israel. First of all, I believe Obama will pressure Israel more than McCain will, but Obama will do much more besides to harm the Jewish people and the Jewish state. The most obvious example of this is the policy the president will take on Iran. Under an Obama presidency, Iran will have carte blanche to proliferate nuclear weapons completely unopposed. Obama will not curb America's dependency on arab oil, while McCain will to some degree. So under an Obama presidency, more American dollars will go right into the pockets of the terrorists.
In some ways, McCain may not be significantly better than Obama, but for Israel, defeating Obama is crucial.