It's not all Democrats, but instead it's the far-left democrats such as Barak Obama and socialists like Sanders and Alexandria Ocasio-Cortez. It's basically part of globalism that's being pushed by George Soros, Angela Merkel and several other leaders of Western European countries such as France and Belgium.
It is a dying idea that was implemented to supplement the populations of various countries that have negative birth rates (i.e. people are dying faster than they're being born). We were lucky enough to avoid most of it here in the U.S. by not electing Hillary Clinton. Pay attention to western Europe news as the idea of open boarders and globalism has allowed Germany and Belgium to gain a huge amount of power and wealth, and they're not likely to just give that all up. Literally, Germany has been able to become the head of Europe without even firing a single round. Now as countries such as Italy, Poland, Hungary, and several other Eastern European countries denounce globalism and open borders, they pose a huge threat to Germany's power in the EU. This could very, very easily lead to WW III.