Not so much a threat to the progressive worldview, but a threat to progressive politics having a positive impact. From what I understand, the 'dirtbag left' consider themselves the gatekeepers of progressive politics, and do a good job of alienating as many other people as possible. The Trumpists of the left, sounds like a reasonable description.

Some argue they were a big part of the reason why Bernie's campaign was unable to find much traction outside his core. Of course they blame it on others, even calling other progressives, like Elizabeth Warren, traitors.