If you, for a second, stop considering migrants as unwanted intruders and consider them as human beings like you and me, this makes in fact eminent sense. Leaving aside the class of highly paid Expats, most migrants constitute what used to be known as Lumpenproletariat, the underlings that are the carrier of revolutionary change. That change is no longer a regime-change on the national level brought about by a revolution, it is a change on the international level, corresponding to international inequality, which has become a problem more pressing than national inequality.
In this scenario, the Western left is actually part of the exploiting class which is now joining forces with racist elements on the far-right to fight off the new Lumpenproletariat. The decline of the left is largely due to this contradiction. In the face of today's change, the traditional left, instead of fighting for the underdog, focuses largely on defending its own privileges, just like the capitalists of old.