"Their America" my ass.
It's not just their America. The rest of us have a right to be here too. Have you noticed how they always talk as if everyone else simply didn't matter?

It's not that hard to figure out, Robert. They feel they live in a meritocracy, and of course, they are obviously the meritorious. So if their lives haven't quite worked out the way they wanted them to, or if they feel threatened, then the danger to their well-being/prosperity/security must be external. Otherwise, they might have to admit that either they are not meritorious, or that life is not a meritocracy, that social forces and even blind luck might play some role in individual success or failure. Either way, their claim to exceptionalism vanishes. You've heard the litany, "they are going to tax us, they want to plunder our stash, they are out to take away our freedom, they, they, they, wah wah wah...". And when the hated liberal is not a destitute looter, but perhaps even more affluent and successful than they are, it is particularly infuriating to them. Then he is a hypocrite, because he is just like them (or the way they want to be).

If you listen to conservatives, everyone who isn't one is lazy, stupid, evil and treacherous; the liberal coddles criminals, doesn't take care of his family, hates his country, actually likes being taxed, and so on. I'm sure you've heard the entire catalogue by now. Listen to conservative rhetoric and their enemies are not just mistaken or misguided, or even misled: they are positively evil. And you will notice the hated liberal, although obviously inferior, barely capable of feeding himself, is always right on the verge of "taking over", that the subhuman, inferior half of humanity that represents all sloth and evil is about to displace them from their earned place, and that conservatives not only have a right, but an obligation to put a stop to it, no matter what it takes.

It's sort of like racism, except the hatred is not necessarily directed at a genetic subgroup, or an ethnicity, or a religion or nationality, or even a competing or rival social class. It is based on an absolute conviction of the innate inferiority of the way other people think. It is a form of philosophical bigotry.

If you're looking for historical reasons for the development of this curious mentality, then I think it can be traced back to the end of the middle ages. After the old dichotomy of serf/noble started to erode, the new middle class of artisans and merchants became more and more prosperous and influential. This new class was a progressive force, but it feared the peasantry, who they felt craved their newly acquired wealth. It also felt oppressed and despised by the aristocracy, it envied them and sought to replace them at the top. Eventually, some of them did.

It took a long time to overthrow the nobility, centuries, and bloody revolutions in England, America and France, but vestiges still remain today, although it is for the most part no longer hereditary. As for the peasantry (the working class), it now has many options to move into the middle and upper classes. But traces of that old dynamic still persist.

The conservative is a relic of the ancient medieval burgher, the man who left the land, came to the town and learned a trade, joined a guild, and eventually became a merchant. But he has forgotten those roots, and the historical and social changes (not to mention bitter struggle) that made it possible. As far as he is concerned, he's made it there on his own, or has not been able to because the Eternal Liberal has held him back. So he fantasizes about some idealized past that never was, and blames others why it still hasn't materialized. And to this day, he still fears the poor, and envies the elite.