This is true, and unfortunately it isn't only the Democrats. Well over half of Congressional Republicans are on the same open borders and amnesty page as the Democrats.
The national question - our sovereign right to decide who does get to live or vote here and who doesn't, is probably the most important issue facing America. Not surprisingly, it has become a litmus test for one's standing relative to the internationalist political establishment. If you're concerned with the national question and favor immigration restrictions, you're an "extremist." In contrast, as long as you're in favor of amnesty and open borders, there's room for disagreement on almost everything else - you have the credentials necessary to be member of the elite political establishment, or at least one of their lapdogs.