From the Guardian:
Last year I pointed out that open borders is, traditionally, an unhinged anarcho-capitalist and laissez faire libertarian idea (here and here) and mass immigration is the last fraud of neoliberalism (see here and here).
Of course, this is the elephant in the room. Few people on the left want to talk about it, because there is a Stalinist culture of political correctness on the left on this issue.
Quite simply, a viable left in the 21st century must be strongly opposed to open borders, because the issue is causing such voter discontent (and not just in Europe) that people will be driven to the populist right and far right in larger and larger numbers (here, here, here, here).