And its also the source of the change in the overall Democratic coalition; the Democratic center-right that has been (and remains) the dominant faction of the party hasn't moved an inch, but the party as a whole has moved because the segment further left has grown substantially, mainly by mobilizing the previously disaffected.
The democrat party essentially is the mainstream American left, that's what people call the left, and that was the topic of this thread ("why didn't the Democrats beat Trump?"). You may not like it or you may have your own definitions of what left and right are, but in this context it is what it is.
No, it is the fucking left.
> The democrat party essentially is the mainstream American left
The US has no political party named “the democrat party”, and the Democratic Party is (as historically each of the two major US parties has normally been) a broad coalition party, the dominant faction of which currently is center-right neoliberal capitalist, not anything even approximating left. The center-left to left component of the party is substantially weaker (though it has grown stronger since 2016, with an influx of the previously disaffected, as I described.)
On a very zoomed out aggregate level, sure, the Democratic Party has changed—and if that’s what you want to talk about, just say that—but the source of that change is the part that isn’t center-right neoliberal capitalist drawing in new blood from outside the party, not a change in the positions of the left (or, for that matter, a change in the position of the dominant faction of the part,y, either.)
If you use “left” to refer to a faction that (1) is largely seen as an opposing force by those who identify as “left”, and (2) largely sees the “left” as the label of an opposing force, and (3) where even you admit there is a much clearer term for what you are actually referring to... Well, maybe you should reconsider your terminology.
The only thing the American "mainstream left" did in the last decade is grow from a completely insignificant size, on a national scale, to a slightly less insignificant size, through a subset of the political disaffected becoming engaged (a big catalyst for that being Bernie Sanders 2016 primary campaign; DSA membership shot up, IIRC, more than 10-fold directly after that.)
The set of viewpoints in that group didn't really change all that much, nor did the set of viewpoints in the actually mainstream groups left of the GOP (which themselves are not actually left, but center-right pro-capitalist.)