> Leftists fought and died for the right to elect these co-opting parties - was it a mistake?
Some would say that, but I expect most would see the unions and their parties somewhat more objectively: as the product of historical development, contradictory formations propping up capitalism, but also responsible for improving the condition of the working class within the confines of the system. I think they'd react with more customary stoicism rather than desair and regret, much like William Morris' John Ball who "pondered all these things, and how men fight and lose the battle, and the thing that they fought for comes about in spite of their defeat, and when it comes turns out not to be what they meant, and other men have to fight for what they meant under another name…"
(Let's overlook the confusing statement that "the thing they fought for comes about in spite of their defeat" and the anachronistic omission of the many "women who fought and lost" the battles; the text still evokes for me those Marxists and anarchists whose sacrifices fell well short of their loftier aims.)
The contemporary American poet and writer Wendell Berry makes the point that "protest that endures...is moved by a hope far more modest than that of public success: namely, the hope of preserving qualities in one's own heart and spirit that would be destroyed by acquiescence." Berry doesn't understand that confidence in the ultimate victory is what really powers social movements, but it's a good statement of what makes a minority of disappointed leftists carry on in the dog days of defeat.