No white supremacist is going to prevent the southern hemisphere from flooding the old dying, disappearing northern hemisphere. There is simply nothing to do about what's going on, just like it was impossible for the Romans to prevent the "barbares" from flooding them. South americans will replace north americans just like muslims will replace europeans. Some of the white men will survive and mix, many will simply disappear.
Also one day might happen when someone with a darker skin color might help you survive an event while people with the same skin as you will abandon you. Life is just way more complicated than that.
