I would imagine I share the experience of many boomer leftists in that pretty much my entire conscious life has been dominated by the awareness that the radical left 1) was marginal, exerting no appreciable influence on policy and 2) that it was a bad thing that it had no influence.
My first experience of this was during the Vietnam period when the left took the position that we had no business invading other countries and that we should immediately withdraw all troops from Southeast Asia. As is by now well known, this was rejected by the bipartisan Washington consensus which took for granted our right to intervene in the affairs of other nations-albeit with a liberal wing sometimes raising questions about the wisdom of doing so. The position that the war was in violation of international law and a moral atrocity, while widely accepted in the general population, was almost entirely absent in policy making circles- consigned to the radical left margins.
In subsequent decades, pretty much every major political issue presented a recapitulation of this arrangement of forces: A bipartisan consensus promoting objectively criminal and immoral policies on welfare reform, crime, environmental protection, nuclear weapons, military budgets etc. with the radical left functioning as the voice of sanity and rationality in the wilderness.
It therefore came as a bit of a shock to find that now after a half century of conscious political awareness, this longstanding equation no longer held. This came to a head was during the election of 2016 when the radical left had associated itself with two objectively insane positions. One was that it was “under no obligation” to prevent the election of Donald Trump. The left (or “left”) thereby became one factor among those responsible for the tragedy which we are now experiencing, the denialism of the left along these lines notwithstanding. While this colossal error shouldn’t be forgotten, it is in the past. On the other hand, another position taken by elements of the radical left continues to matter. Namely, its having smeared Sanders as a “sheepdog” candidate functioning to channel progressive energies into an unreformably corrupt Democratic Party where the left will have no influence.
Absurd as it was when it was first proposed, the bankruptcy of the Sheepdog thesis should by now be obvious to anyone with a minimal degree of political awareness. Most notably, the Sanders wing has already exerted considerable influence on the Democratic Party with Sanders formerly pie-in-sky policies, single payer, $15 minimum wage, marijuana legalization, now finding their place on the agendas of neoliberal front running candidates, Harris, Booker and Patrick. Does that mean they should be supported? Of course not. The left should be opposing them vehemently based on the assumption that they are only paying lip service to the Sanders agenda and will quickly drop it once in office. But doing so requires, “participat(ing) in the internal life of the Democratic Party”, something which the radical left has declared a fatwa on, thereby consigning itself to irrelevance at best, and at worst, making itself an obstacle actively hostile to taking the direction which is necessary to succeed.
The bottom line: Insofar as the radical left, or the “left” has been consistently antagonistic to the Sanders mobilization, it has, for the first time in my life, put me in the position where I am grateful for their being as dysfunctional and marginal as they have always been.