Its not so much the organized militias, more folk acting on their own who claim a white supremacist or related motive. Much more deadlier than Islamic extremism or the almost non existent deadly leftist violence over the past few years in the US. And being against immigration is not the issue. Promoting the idea that the white population is being systematically replaced by a global Jewish elite is rather more dangerous. Or even as Trump has done in his rallies, suggest that "Islam has a problem with america" and needs to be subsequently "dealt with". And that shit is not confined to the borders of the US, as yank cultural capital travels far and wide, including in far right circles. Even among the relatively small extreme right community in Ireland today, you can see the language and ideas of American white nationalists imported wholesale.
I'm an American, I'm a bit more ambitious for the country than you are. Doesn't happen everywhere. A country with our wealth should be able to get our heads round it and take evidence based measures to stop it happening.
I'm not sure I agree with you that anti-immigrant sentiment is organic. There's a vested interest in those who benefit most from the status quo to ferment that kind of hostility, and you can see it in the media and in political rhetoric. It distracts people from analysing structural issues in society and who is benefiting. Trumps a master of it, classic divide and rule tactics employed by elites for centuries