I dunno...
Seriously, I grew up when Stokely Carmichael and H. Rap Brown were running around trying to keep up with the Black Panthers, and bigots like David Duke were still acceptable mainstream politicians in large areas of the country. So you will forgive me if I don't find the irrelevancies of David Duke and Louis Farakahn very threatening.
The knownothings thought that the wave of immigrants coming in back then would destroy the U.S. The wave of immigration from Southern Europe from the 1880s-1920s also was predicted to destroy America.
Now, I may get defrocked as a liberal for saying this, but I think the U.S. will survive just fine even if Bush gets reelected to four more years. The United States has faced challenges before. I just don't share your pessimism.