Over at the Society for U.S. Intellectual History, Ben Alpers observes the relative lack of attention paid to the fact that the Romney-Ryan ticket is the first from a major party without a Protestant candidate. He correctly points out that a Mormon-Catholic alliance, supported by a largely Evangelical base, represents a dramatic shift from an earlier era, when both anti-Mormon and anti-Catholic sentiments were integral to American nativism. The two groups have had a tough time with one another as well, as described in Matthew J. Grow’s colorfully titled 2004 Church History article, “The Whore of Babylon and the Abomination of Abominations: Nineteenth-Century Catholic and Mormon Mutual Perceptions and Religious Identity.”
The question remains: if we have come so far in our religious tolerance that these former enemies can become allies, what are they allied against?” It seems to me that this sort of decrease in tension between Christian sects is a circling of wagons against what they imagine to be “the secular.” Doctrinal differences can be overlooked if ideological stances match up. While Romney, Ryan, and their Evangelical supporters have very different soteriological beliefs, they share a particular American Christian worldview that transcends sectarian divides. In the upcoming months, expect an increase in salvos leveled at the “secular left,” which would include the media, liberals, and of course, Barack Obama. The blending of Christianities in the Republican ticket allows the GOP to present a unified front against not only this imagined godless left, but also against Islam, which did not get an invitation to this breakdown of sectarianism.
(ir)religion in america 
I think this is an interesting article, Charles, and I think it is a result of some strange developments. I like Susan Rudolph in The Sacred and the Sovereign: Religion and International Politics, “From a Universal Religion to Universal Religiosity” (a bit dated, but an interesting idea). These days, I think most churches silently and begrudgingly accept that there will not be a monarch that will enforce Westphalia and have everyone in its realm follow his reign. Therefore, they move to a larger sphere of ethics and worldviews on what they see as “the important issues” which tend to be a long road to the right’s social ethics (housed in language of culture war and so on). This can be done ecumenically in many senses so long as they don’t disagree on the “really important matters” – abortion, gay marriage, evolution. The meaning of Jesus and the central aspect of faith systems seem to be negotiable.
In some churches, I think this same thing is happening. The Lutheran Church, Missouri Synod has gone to level of being so interested in these social “truths” that they have become the single determining factor for a denomination. I think it is more likely for them to ordain a pastor who does not believe in some tenets of the Lutheran Confessions (the normative book of faith since 1580 for all Lutherans) than one who believed in gay marriage.
The focus on these “key issues” is, I think, one of the defining features of American Christianity as a socio-political entity today. Fred Clark at Slactivist (http://www.patheos.com/blogs/slacktivist/) has been writing some excellent pieces on Christian tribalism–the phenomenon you describe of requiring adherence to ideological points rather than doctrine. His years-long project of ripping the Left Behind novels to pieces is also worthy reading.