I am so left wing that you would probably be hiding behind a wall if you saw me coming. But here's the thing. I, like many on the left, am not pro-immigration. I am simply not anti- immigration either. And I do my very best to not be racist, even unwittingly. Most of us are in favour of a nuanced debate about what immigration could look like, just as we are in favour of a nuanced debate about benefits and all the other controversial subjects. I was brought up, as many of us were, that discussion and argument (i.e. debate) were the way to discuss political issues. That is not what happens on this site. Immigrants are not responsible for most sex crimes, they are not terrorists, they are not all Muslims, they should not be left to drown in the Channel, there are no shops displaying signs that say no English allowed. There are problems with immigration. There have always been problems with immigration. The Celts thought the Romans should go home (they were possibly correct!). There are problems with the "British" (whatever they are).
If you knew what a great many on the left actually think, you'd probably be gobsmacked. But "left" is just another insult that is bandied around, just as "right" is.
You may not agree with people on the left (or you might be surprised to find out that you do), but they come to their positions through discussion and debate - that is the hallmark of the left, like them or not. To be fair, I do not like the politics of conservatism, but I recognise that many have come to their position through equally genuinely held thought and discussion. And I can debate with and deal with that. The true "right" - Farage and those to the right of him - look at what they do. They do not debate. They do not argue their position. They come up with slogans and rants - no rationale at all.
Anyway, I digress - racism is not acceptable full stop, whatever your legitimately held views, because it derives from hatred. There is no room for the politics of hatred.