Witherington asserts that Mormons deny the sufficiency of Christ's death for salvation, but I don't think that is right. It is right that individual Mormons sometimes speak of works as if grace were either irrelevant or merely a means for filling in the gap left over after our works have done what they can. But that isn't an official view among Mormons. (After all, there is an official view on very few doctrinal questions.) And the Book of Mormon quite clearly says otherwise. For example: "[I]t behooveth the great Creator that he suffereth himself to become subject unto man in the flesh, and die for all men, that all men might become subject unto him" (2 Nephi 9:5).
I've argued (here, for example) that "Mormon talk about obedience does not deny the necessity [or sufficiency] of God's grace. Instead, as the Book of Mormon teaches, 'It is by grace that we are saved, after all [i.e., beyond all] that we can do' (2 Nephi 25:23)." (See also here.) Contemporary Mormons talk about grace fairly often, but we aren't good at explaining it. (There are only slightly more than zero Mormon theologians.) In spite of that I would argue that most Mormons believe, at least implicitly, that Christ's death is sufficient for salvation.
Mormons do believe, as Witherington says, in a form of theosis, though whether that takes the form preached by some in the 19th century (the view ascribed to us by Witherington) or something more like what one finds in Greek Orthodox belief remains an unsettled question.
The fact, however, that the question is unsettled means that one cannot say definitively that Mormons believe that those who inherit eternal life will be gods in the same sense that the Father is God. Nor can one say definitively that the Father was once a human being like us. About such beliefs one can only say that some believe it and some don't. The Church hasn't taken a position on the questions, so we are left to ourselves to speculate.
My personal views on such questions can be read here and here, with a related piece on God's transcendence here.
It is perfectly reasonable for Ben Witherington to disagree with us about the Trinity, about human nature and destiny, about latter-day prophecy, about a variety of theological questions. It is less reasonable for him to insist that those of us who disagree with him, even after recognizing his biblical learning, are deceived. That is equally as unreasonable as it would be for an Orthodox or Jewish theologian to insist that Witherington's understanding of theological topics about which they disagree can only be the consequence of him being deceived or deluded.
But all of this is irrelevant to the question of who I should vote for, Romney or Obama, though unfortunately Witherington puts the issue in that context. To introduce the question as he has is to make religion a criterion for the office of president, even if the criterion is only a thinly veiled hint. That's neither illegal nor unconstitutional, but it is wrong.
Were a Muslim to run for office who is an honest person of integrity and who was qualified for the office, I would not think I should vote for his or her opponent merely because the other person was a Mormon or Christian or, heaven forfend!, a Jew or Buddhist.
I'm not likely to vote for Romney, but if I did it wouldn't be because he is a Mormon. If I vote for Obama, it won't have anything to do with his religious beliefs, except insofar as those are part of what demonstrate his honesty and integrity.
The question is not "What is this candidate's religion?" but "Is he honest and trustworthy?" and, perhaps more important, "Are the policies for which he argues ones that will benefit the United States the most?" Even if Witherington were right about Mormon belief, which he sometimes is and often is not, that wouldn't be a reason to vote for or against Romney.
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