The same problem dooms both the Democrats and the leftwing insurgency within the Democrats: Democrats love and trust America’s institutions.
This love of institutions makes it impossible for the Democrats to consistently do well- because trust in American institutions is at an all-time low in the electorate generally. Any party which says American institutions are good will have trouble winning elections.
And the Democrats love of institutions it makes it impossible for the left of the party to do well within the party, because the left is inherently opposed to, and opposed by, the dominant institutions of America.
Most obviously this applies to the media. The Republicans have been trained to distrust the media- even their own media like Fox. This made it possible for Trump to win the Republican primary despite media opposition. Poll after poll shows that the Democrats are far more trusting of the media, which has allowed the media to hamstring Bernie’s campaign.
While the centrism of the Democrats makes them ideologically slightly closer to the left than the Republicans, in their emotional valence the left has more in common with the Republicans than the Democrats. This mismatch makes the already difficult job of gaining leadership of the Democratic party that much harder.
It cannot be overemphasised how critically bad it is that America’s further left major party is considered the more pro-establishment party. How is the left meant to tap into discontent with politics and business as usual under these conditions? Even for the leftists completely disconnected with the Democratic party, the pro-establishment malfeasance of the Democratic party effectively smears the reputation of the left, making us appear tied to America’s godawful elites. Right now the American political spectrum stretches from small c conservatism (defence of existing institution) to big C conservatism (reaction). The left has been imprisoned by the former program.
A concerted campaign that aims to convince people that the media is untrustworthy precisely because it has ties to rightwing institutions is thus necessary. Such a campaign would tie together the right attitude and emotional valence (distrust) with the right (that is to say left) political and social content. If done well, such a campaign could reach and draw leftward both:
A) Apolitical types with low trust in the media and other institutions. This campaign will give them a tool to conceptualise this distrust in leftwing terms.
B) People with basically social-Democratic or even mildly socialist instincts who, nonetheless regrettably trust the media at the moment.
If such a campaign were wildly successful, it might simultaneously free dem voters from the control of the party establishment, and help detach the Democrats from their fundamentally losing “nothing is fundamentally wrong with America” attitude. In other words, it might not only help the left win victories within the party but also the party win victories in elections. Alternatively, if the left chooses to abandon its current strategy of trying to seize control over the Democrats, distrust of the media will make it easier to split off a chunk of voters.