Why the "Left" Always Seems to Lose...

The American "Left" is a chimera; it's an "idea" more than it is a "movement": An "agenda" without a "constituency." And it is unlikely to be transformed anytime soon.  That's because the "Left" is (too) fragmented to effectively resist the concerted might of the Right, consisting as it does of a fragile and fractious amalgam of the counter-hegemonic interests, many with contradictory aspirations and expectations; and all of which are conditioned to struggle over the scraps thrown from the plutocRats' table. 

So, unfortunately, this will likely continue to be the case until they can plainly announce themselves as the ANTI-WHITE-SUPREMACY party, and have that single principle around which to organize.
This has been the secret of the Right's recent successes. The GOPhux have a single, simple set of organizing principles: 1) Keep The Nigras ("Undesirables") Down; 2) Preserve White Supremacism, and 3) Privatization. 

They have the additional advantage of being the Party of the PlutocRats and CorpoRats, and thereby imbued with the superior social status, enabling them to pull shit for which, if they tried it, the DIMs would be laughed into the Potomac. For instance, try to suppress your laughter at the idea of the Dims shutting down the Government, for ANY reason. Imagine the Dims, even with a majority, refusing to hear a GOPresidential SCROTUS nomination.
"White Power" will likely persist in the USofA long past the end of the White demographic bulge, because it is, I believe, extremely unlikely that People-of-Color will have the political power to wrest the financial hegemony away from the White establishment which will, in any case, bend every effort to turn the DIM fragments against one another--which works just about every time..