When in US history have we had more than two major parties?
Be very specific.,
That’s pretty obnoxious – why don’t you be specific about when we have only had two parties?
You’d have to include elections where there were basically three parties: 1848, 1856, 1860, pretty much every national election 1880-1900, as well as third or fourth party candidates in 1912 (Eugene Debs!), 1924 (Bob LaFolette!), 1992 and 1996 (Ross Perot).
That has never happened, and it’s not how you make the Dems more progressive.
Above I linked to an article about how Sanders moved Clinton to the left, perhaps you could back up your assertion?