Your circular logic isn't making any sense.
The Dems think that building a wall to force people to come in thru controlled ports of entry is:
-wrong
-immoral (even though they supported it before Trump made it a campaign promise, so now they're against it)
-too expensive (even though they would spend billions to provide healthcare, education, and other social programs...some of which the actual US citizens have trouble getting)
-ineffective (even though the experts, local law enforcement, and local citizens know its effectiveness, but are ignored)
They want to abolish ICE. In the mean time, these sanctuary cities have laws which prevent local law enforcement agents from telling the Federal Agencies that they are detaining non-citzens who are potentially dangerous.
The advocates of sanctuary cities say, "Come here, we have a place for you"...but when Trump says he will send the released detainees to these cities, the advocates suddenly say, "WHOA! Don't dump them here. We don't have a place for them."
So how can you ask "So who exactly is it that is advocating just opening the borders and letting anyone who wants in?" That's the way it is now, and nobody but Trump seems to want to do anything to change it.
