I think this one is saying that the Dems, basically, should understand that their tent is a coalition between those on the left (lefties) and those in the middle (centrists). I would go a step more and suggest that the Democratic party is a coalition of a LOT of different groups which have a single goal - get rid of Jackass. I would suggest that, even without the Jackass thing I believe all under the Dem Tent have some basic agreements that are not embraced by the Republicans. Some might be; regulation is necessary, there are societal undertakings more reasonably done by society than private industry (public schools, fire department, police department, libraries, healthcare, etc). Just those 2 should be enough and I am sure there are more than that.
The problem, I believe, is in some groups determined to have their way or the highway. I also believe this has nothing more to do with than individual egos and little else. If we are to move forward then we ALL gotta move forward together without a bunch of bicker. If there is a problem sit down and work it out. There are some basic facts and, in this regard, forget 3rd parties and other fantasies. Not gonna happen. If you want a place at the table state your case but do not, for instance (as happened in 2016) vote for the opposition which is against, basically, everything you believe and hold dear. (I think is a bit like sacrificing integrity to make one feel better?)