I think one of the main reasons we have allowed politics to be infiltrated by religion in the past couple of decades is because Americans as a people really have no recollection of being involved in a holy war.
Some of our ancestors came to America fleeing religious persecution, we all learn that in school fairly early on. But we don't really identify with the Puritans or Quakers, or understand how they were oppressed, and nobody but the geneaologists remember the Palatines. The Lutherans and other Protestant denominations and sects also came seeking freedom to worship as they chose. But all that's ancient history to us, hundreds of years in the past to a society that has trouble remembering what happened a handful of years ago and who shouldn't be allowed back into positions of responsibility.
We see Northern Ireland, and Bosnia/Serbia/Croatia from afar. The Near and Middle East have always been a powder keg. Darfur is a distant tragedy. The Crusades are just stories out of ancient history. Most don’t know or remember that hundreds of thousands were killed throughout history over religious differences or religious power struggles: In the Holy Land, in Europe and in Asia, over thousands of years. The Inquisition, the Reformation, the translation of the Bible into English, all sparked horrible deaths and bloody wars based on religious differences, some of them seeming ridiculously petty.
Now, America has done some persecuting. We followed the European precedent of using religion as an excuse to steal from and inflict actual and cultural genocide on the First Nations. We harassed and chased, then exiled the polygamous Mormons to Utah. We periodically harassed minor offshoots of established religions, periodically harassed Catholics. But Americans as a people have never been the victims of an overt religious persecution. We don't viscerally understand how dangerous it is to allow religion to become not only a public practiice instead of the traditionally private one, but actively involved in politics and government. None of us had family members burned at the stake for heresy, imprisoned or killed for translating the Bible into English and distributing copies. We don't see the relationship and the almost inevitable bloody conflicts that happen when religion gets loose in public and begins to accrete power.
Humans are programmed to like being right. It feels good. We want more of that feeling. Some people approach religion in a loving, holy, harmonic sense, with the intent of being closer to their conception of God or the Universe. Others approach it as a power structure. Either of these can feel good and prompt us to seek out and reinforce those feelings. This is where religion gets unstable and is biased toward evolution. If some is good, more is better, that's what our primitive brain tells us. If these teachings or practices bring us closer to God, or give us power over others, what's the next step? And there's always a next step. The next step is the beginning of a positive feedback loop, and positive in this sense isn't good, positive in this sense is active, and accelerating. How can finding a way to be closer to God not be good? How can one pass up the opportunity to be more right, more righteous? Either closer to God or more powerful than those presently closest. And since matters of faith are, by definition, not subject to testing and definitive proof, the person who can convince others of their point of view gathers power or the perception of holiness, the prize. It's very hard to put the brakes on this process after the purity trolls get loose, burning at the stake didn't slow people down that much, if the soul is perceived to be at stake, something beyond the physical body or existence, if the stakes get that high, things get ot of hand very quickly.
When the Abbot of Citeaux, Arnaud-Amaury, (a monk of the Cistercian Catholic order), was acting as a general in the Albigensian Crusade(starting in 1209, the only Crusade within Europe). His army surrounded the city of Beziers, in France. His lieutenants asked how they were to tell the Cathars from the Catholics, since Catharism was a religion of choice, not of ethnicity, and they all looked alike. He uttered the order which has outlived the memory of his name and a version of which is still used today as a macabre joke, ‘Caedite eos. Novit enim Dominus qui sunt eius', (Kill them all, God will know his own). So even being of the ‘right’ faith wasn’t protection from fanatics. In pursuit of immortal souls. Americans don't grok that. We don't conceive of the possibility that we will be on the wrong side of the religious persecution. We don't grasp the danger in allowing religion to accrue political and secular power. Even with all the examples around the world, ancient and contemporary, we still don't get it.
Those who cannot remember the past are condemned to repeat it.
George Santayana