Scabbing is an ugly word. It refers, in the labor movement, to individuals whose personal values or perceived interests are more important than the collective interest of a given time and struggle. The individual crosses a picket line declaring him or her self as an enemy of, and traitor to, the the collective struggle, typically a strike.
The labor movement is a lot weaker these days, and strikes less frequent, so the term has lost some currency.
It may seem unfair to apply the term in a political context. But I can think of no better one to describe two trends, at first appearing to oppose one another, but actually united in placing narrow personal interests, pet peeves, and "principles" above the common struggle.
The first is the trend that sat out the presidential election, claiming, absurdly that there was no difference worth noting between Trump, a fascist and Clinton, a liberal. That trend helped elect a fascist. Is "left wing scab" an unfair term there? I don't think so.
The second trend is the one that is now blaming Sanders, and a "class" approach to politics, for Clinton's loss. Also absurd, and, since Sanders is now THE leading voice in the anti-Trump movement, a move to divert the anti-Trump, anti-fascist struggle into infantile leftist masturbation sessions so characteristic of the US left -- and a longstanding signature of its impotence.
Both of these trends are motivated, like the scab, by fear. On the "left", its the fear having to live up to progressive principles -- including socialist ones -- and put it on the line to millions of people like Bernie has done. Of the folks shooting off their mouths against Bernie -- you don't see them running for shit. O no -- their JOB is to point out "flaws", declaim against the "white working class", shed copious tears of admiration for the passing of Obama. But you know the little white thing on top of chicken shit: that's chicken shit too.
It's a primarily white privilege of underestimating the fascist danger, because its consequences won't fall on you. But they will fall on you. God cast his judgement for the offense of slavery on both North and South. Its a fear of the hard work of building working class unity, without which there will be no defeat of the ultra-right. Some think the ultra right rise is unrelated to longstanding inequality -- that's their excuse -- like the scab who says: I want higher wages, but those militants are threatening MY job.
Cowardice hides behind many tropes -- it takes a genuine fascist threat to get them to all stand and be undressed.
The cure for scabbing is confession. But if you head too far down that road without correction, you can get lost forever