The fear is that we are "losing our country" and that is premised on the idea that there are people who are not really American, or not particularly American taking the upper hand. A lot of them are not white in the usual sense of the word.
Lots of them are brown, so let's do what we can to throw them out. Lots are black and though not many of those vote, too many do. Of course the gays aren't white in the way we are.
We are also scared that because some people want to put limits on the availability of firearms so we don't end up with the drug dealers shooting it out with the cops and kids aren't going to school packing and doing foolish teenage things, the government may be plotting to take all our guns away from us, which would interfere with our god given right to bear arms. So we exagerate that threat like we exagerate the threat that all the brownies will put the PRI in power here---never mind that the Mexicans coming here hate the government in Mexico.
We are also afraid that the old time religions are dying out, so we start howling everytime some atheist whines about the creche at the courthouse (as opposed to at the churches, which even the atheists are ok with) and claim that there is a war on god rather than face the fact that the war is by the knuckledragging wing of christianity, which thinks the dinosaurs were around about 5,000 years ago and that the giant flood described in the Gilgamesh Epic actually happened and covered the entire world.
Mostly, the southern conservatives are worried that their society will be overwhelmed by the blacks, whom they believe are largely uneducable and a drag on the white man.
And they are right. The country is changing and getting less white, just as it was 150 years ago, only now it is not the Jews, or Italians or Germans that are scaring the white folks, it is another bunch.
Think about it: why the hysteria (you will admit it was hysteria, wont you?) about Obama being born in Africa and being a muslim? Is this so different from the hysteria that touted Clinton as a red? Or JfK as a red? Or Eisenhower as a red?
Is this paranoid style not a regular feature of scared people in the US?