Far better to run as Democrats and change the party from within, while of course retaining a separate platform. You get not only the funding infrastructure but the ability to affect matters immediately, in Congress as well as at local and state levels.
Taking a long view, say 150 years, D and R don’t mean much in themselves other than to reflect currently prevailing ideological divisions. I doubt, for instance, that the early 20th century Southern Democrat would find much empathy among liberals today.
Even what “liberal” and “conservative” mean tend to shift around. For instance, few today would associate liberalism with “classical liberalism” (now ironically called “neoliberalism”). At the same time, a Burkean conservatism that recognizes our finitude and hence our duty to care for what we have been given, to cultivate it and let it come into its own, rather than to master it and shape it narcissistically in our own image, finds its strongest echoes in the modern Green movement with its call to “conserve” our environment and its lifeworld.
So if we see the D party as something that’s always in flux, and we appreciate the current moment as one of crisis or fundamental change, why not help the party along to its next destination, where it can be most relevant again? The DSA can and should retain itself as an informal wing. Likewise, the Greens should reorganize similarly and hopefully along the way get rid of narcissistic losers like Stein. One can join both, or either, or yet something else, maybe something new can emerge. The nice thing is then all of these different groups can supply candidates that provide diverse perspectives while at the same time unifying around some very general common goals.
This is all a bit hypothetical – right now the DSA does seem the only game in town in terms of actually affecting things, so I’d tend to look there until another association has really proven itself. Of course the most pressing problem is what to do about the DNC, and they don’t seem to be learning from anything, just rearranging chairs on the Titanic.