Only a relatively small percentage, I would guess no more than 20%.

However, they have convinced themselves (or someone else has) that they are a majority of Americans, and that anyone who isn't like them isn't a "real" American anyway, and that those strangers have taken their country from them by force.

Its not surprising. Most people hang out with people like themselves. If they are in rural areas, they don't get a chance to meet a broad cross section of people in their relatively homogeneous communities. They actually think Mayberry RFD is the REAL USA. They really haven't realized that they are now fast becoming a minority, that rural, white, N European stock are not America any more. Even many people in that community are educated and informed enough to understand this country isn't made up of people descended from the passengers of the Mayflower. Most Americans came in through Ellis Island, or where already living in N America when the Pilgrims got here (the Mexican West), or came in on slave ships. And now they're flying in on jet airliners.

That's why they "want their country back". They're just now starting to realize it was never theirs in the first place.