Bernie Sanders is just the most recent factional candidate

Slate - There Is No Bernie Sanders Movement

This piece places the rise of Bernie Sanders into the context of the challenges to the Democratic party mainstream that have historically come from the left during the primary process. The argument is that he follows the same general "good governance" rhetoric that Howard Dean and Bill Bradley did in 2000 and 2004, or even going back to Jerry Brown and George McGovern. The problem isn't that their messages don't resonate with young, white, (mostly college educated,) liberals. The problem is that this demographic does not make up the majority of the Democratic party. While they make up a greater share of the party than they have historically (due more to the collapse of other elements of the party, cf social conservatives), and are enabled more than ever before with social media and distributive fund raising, they still do not have traction with other parts of the party. Instead, parties are coalitions and Sanders has not been a coalition builder (with the notable exception of Michigan-- an example that is informative). Sanders seems to have the biggest problem with black voters, who have their own priorities and are not motivated by the "get big money out of politics" as a dominant voting issue. In addition, modesty about Sanders is requisite: while he is popular with youth, the youth isn't a dominant voting block, and his energy isn't turning out more voters than 2008's Obama.
The piece talks about the possible future for Bernie Sanders-inspired politics. Bouie takes a look at the Barry Goldwater defeat for the GOP and how they built an ideological core that started to influence organizations, then local- and state-elections, finally taking over the GOP with Reagan. There isn't an analogous playbook for Democrats, but if the idea is that to make "Sanders Democrats", e.g. his ideology into the majority of the Democratic party, a primary is not the way to do it. The prescription is to take the Sanders energy to down-ballot races, local organizations, and to work in different communities over the long term.