I said Romney could never win the GOP primary ...

Five years Romney was running for President. Back then I wrote:

Gordon's Notes: Romney: not possible

In order to win the GOP primaries Mitt Romney has to convince Christian conservatives that he's reversed many of his longstanding opinions. He also, incidentally, has to publicly renounce his religion and be born again as a Baptist ....

I wrote my post around Kenneth Woodward's 2007 NYT OpEd.  Woodward walked through the theological gulf between Christian fundamentalisms and Mormonism -- and he didn't even touch on the more exotic portions of traditional Mormon belief. I didn't believe a former Mormon bishop could win the GOP primary. Perhaps he could win the presidential election -- but not the GOP primary.

It took five years, but it seems I was not exactly right.

I'd like to know how he did it, and how evangelicals crossed that divide. In the meantime, I think some credit goes to GOP voters. Maybe quite a bit of credit -- depending on what they were thinking.

So how exceptional is Romney's religion? Is Mormonism technically Christian? I tried two sources, both were a bit evasive ...

Encyclopedia of Religion and Social Science

... In its Christian primitivism and antinomianism, it was akin to many other "restorationist" movements, such as the Campbellites, which emerged at about the same time in the "burned over district" ...

... in the 1840s, Joseph Smith and his successor prophets began to promulgate a series of new revelations and doctrines that moved Mormonism in a sharply heterodox direction relative to the Protestant heritage from which it had emerged. Since then, mainstream Protestantism, especially the more evangelical and fundamentalist varieties, has generally been unwilling to consider Mormons as part of the Christian family, despite the continuing Mormon claims to being the one, true, authentic church of Jesus Christ, restored to usher in a new dispensation of the fullness of the Gospel.... 

and

Mormon (religion) -- Britannica Online Encyclopedia

Mormon beliefs are in some ways similar to those of orthodox Christian churches but also diverge markedly...

.. Mormons regard Christian churches as apostate for lacking revelation and an authoritative priesthood, although they are thought to be positive institutions in other respects. Smith, they believe, came to restore the institutions of the early Christian church. Although calling people to repent, Smith’s creed reflected contemporary American optimism in its emphasis on humanity’s inherent goodness and limitless potential for progress.

Perhaps it's a bit of a sensitive question. I'd go with technically Christian-related but not Protestant; probably closer to Christianity than Unitarianism or Judaism. Obviously there's a bit of irony if the Obama-is-a-muslim slice of the GOP ends up electing an arguably non-Christian President. (Maybe that's why Romney encourages Trump; it keeps the Birther whackos busy. Left alone they might go in another direction.)

If Romney does win he'll be expanding the religious range of the American presidency ...

Religious affiliations of Presidents of the United States - Wikipedia, the free encyclopedia

Unitarian

• John Adams
• John Quincy Adams
• Millard Fillmore
• William Howard Taft

No denominational affiliation

• Thomas Jefferson
• Abraham Lincoln
• Andrew Johnson
• Ulysses Grant
• Rutherford Hayes
• Barack Obama (previously United Church of Christ)

However, since the list includes four Unitarians (we go there - they evidently take agnostics) the historical record already stretches a good bit beyond "mainstream" Christianity. So President Romney would be unusual, but, theologically speaking, not entirely unprecedented.