I think that pro-immigration policies from centrist and left-of-centrist parties have been a major strategic blunder. Large numbers of people are really unhappy about that and, in my experience, even people who had been reliable left-wing voters often become really unhappy if the number of immigrants (especially poor immigrants) is large enough. I don’t think political parties that support immigration ought to be so surprised when these people vote for someone else - after all, in a democracy the people get to vote for the parties that propose the policies that people actually want.
prioritizing preserving the traditional white-dominant society over protecting their democracy