In almost every religion, there have been larger and smaller schisms between groups of people who interpret the Bible and other texts differently. Christianity started as a group of people who followed a particular set of teachings both based in but at odds with parts of Judaism. Even in the early church there were divisions, conferences where factions of people excommunicated each other. Within the Christian Church you have the Great Schism between the Catholic and Eastern Orthodox Churches, then you have other smaller splits, the Reformation which split Catholics and Protestants, then Protestant churches split into a ton of different denominations.
All of it is over interpretations of texts and traditions, but mainly about power and belonging to the "correct" club of true believers. Fundamentalist doesn't really have much meaning anymore, along with Evangelical.
What we are seeing is the reaction of people who were raised in a time where Christianity was the de facto religion, where the social norms while not explicitly promoting Christianity because of the law, were doing it socially. Think about it, a lot of places still get Christmas off. Even government services are shut down. Not Passover, not Kwanza, not Rhamadan, but Christmas. People are freaking out because there are challenges to the norms now and for Christians, challenging their dominance feels like oppression. They are projecting and they are wrong, but that is how they see it.
They will support anyone they think will maintain or restore the cultural dominance of Christianity.