The use of 'middle class lefties' as a political insult is profoundly stupid for any democratic party right now. Democracy is under threat in the US and elsewhere, yet Labour seems unable to look beyond the current electoral cycle. Assuming they form the next government, in 2029 Labour will likely be facing a populist opposition for whom 'the metropolitan elite', 'the mainstream media', 'lefty lawyers', 'experts', and 'middle class lefties' are enemies because they bring a degree of dispassionate analysis rather than emotion to debates.
The resort to emotion is the calling card of the demagogue, who wants his/her listeners to distrust others so that they heed only his/her words. Anyone who is not 'one of us' is to be distrusted, ignored or shouted down. With a careless disregard for how it might play out, by isolating 'middle class lefties', Labour is reinforcing the framing that will endanger our democracy.
To borrow from F Scott Fitzgerald: 'They were careless people, Keir and Wesley – they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made.'