I'm not defending Trump and I'm not attacking Obama. I'm pointing out that Democrats are moving further to the left, and that the myth that Trump constitutes some radical rupture in American politics is concealing to them the fact that there's actually much more continuity between Trump's immigration policies and Obama's. Subsequently, there's a double-standard to their position.
The fact that Trump's child separation policy is so cruel colors the way in which the rest of border enforcement is perceived, so that more mundane features of the immigration policy are seen as extensions of Trump's unique cruelty. For example, some on the left have been criticizing Trump and calling him a fascist or a nazi because of the living conditions in the detention centers (specifically, the fact that detained migrants are held in chain-linked fences, or cages). But this system wasn't constructed in the past year and a half since Trump became president. It was built a long time ago. The thing that they find so troubling now is only under increased scrutiny because Trump is doing it. In other words, this very strong reaction against Trump is pushing Democrats further to the left, so that the left is more and more frequently presenting arguments that were once only right-wing caricatures of the left's arguments (e.g., some one the left are arguing that borders are inherently inhumane, and, therefore, the only moral position is to give up on border enforcement altogether).
It's easy to describe border enforcement as fascistic. It's easy to look at pictures inside those detention centers and to say that they're as morally offensive as concentration camps. It's almost tantalizingly easy, and seductive: after all, there is undoubtedly a semblance of truth to the comparison. But as soon as you do that, you're adopting a view which is entirely impractical and unrealistic.