The issue is the narrative that immigration is fine and desirable as if that was an universal truth and if immigration should therefore be encouraged as a matter of course.
I think this is a big mistake of some of "the left" to go down that route because it is not fact and people are increasingly pushing back against that narrative (judging by election results throughout Europe and US)
The topic does not have to be partisan as it touches on core aspects of society and national culture, which ought to be discussed and debatable in a non-partisan way.
My theory is that this an evolution over time of anti-racism movements that started (in Europe) mostly after WWII: immigration -> anti-racism -> accept diversity -> diversity is good -> promote diversity -> immigration is good -> promote immigration.
The topic does not have to be partisan as it touches on core aspects of society and national culture, which ought to be discussed and debatable in a non-partisan way.
My theory is that this an evolution over time of anti-racism movements that started (in Europe) mostly after WWII: immigration -> anti-racism -> accept diversity -> diversity is good -> promote diversity -> immigration is good -> promote immigration.