Mexicans in Denver are protesting Columbus Day and treating Columbus as a villain. To any American, he is, by definition, a hero. No nation can survive hating its past. It also serves to prop up the welfare state apparatus by expanding demand. In schools, immigration causes overcrowding and learning in multilingual classrooms.
Immigrants are over-represented in the criminal population.
The problem is not just assimilation. High numbers intrinsically defeat immigration, both by making ghettoes viable and by creating political pressures for multiculturalism. And if the problem is that the means of assimilation are broken, the obvious answer is a moratorium until they are fixed. Immigration will force us to accept social mores we abhor. Moslems, for example, will vote to allow polygamy.