Fair enough, so rather than the OP's claim that this is some "growing faction in the Democratic Party" it's just a few people on the fringe.
I think the OPs statement is an accurate one, technically speaking.
RootsAction started its online activism at the beginning of 2011, and has had near-continuous growth ever since. We grew to 100,000 people by the end of 2011, and 200,000 by the end of 2012, 500,000 by the end of 2014, and over 640,000 in 2015. Today, RootsAction has 1.3 million active members.
100k members in 2011.
640k in 2015.
1.3 million currently.
So I guess that would constitute a growing faction.
However, I don't know exactly how that would translate to voting patterns.
A lot of times, simply having a membership to an activist organization doesn't always equate to "wouldn't vote for another candidate push comes to shove". For instance, PETA has over a million members in the US. And I'm pretty sure their "perfect party & candidate" would be one that advocated for banning zoos, hunting, leather, and red meat sales, but I suspect that many of them still went ahead and voted for whoever Democrat was on the ticket.
With 1.3 million members in RootsAction, I don't think that would be quite enough to get a major US political party to partially gut and revamp their policy platform at a national level. They may have be able to have some success at a regional or state level depending on the locale.
That's where, in politics, multiple playbooks are required.
There's one playbook for: Mayor/City Council/House Rep
There's another for: Senate/Governor/President
The strategy that can produce a landslide victory in a solid blue/red house district or very progressive/conservative city, could entirely flop if that exact same strategy were used in a Senate or Governors race.
A good example to highlight that would come from my own state of Ohio.
Jim Renacci had back-to-back landslide victories when he ran for House Rep, when he tried his hand running for Senate, he lost by 300k votes, and when he decided to take a run for Governor, he couldn't even win the primary. I suspect the more partisan members of either party (Like the AOCs and Jim Jordans of the world) would likely run into the same challenges if they tried to run for Senate on the same playbook that got them their House seats.