Well, other than slavery, I don't know what the historic positions of the RAT party were. But I do know as far back as FDR with his Socialist Security, that the Socialists lightened up on running candidates, as the RATs were doing a fine job of promoting the progressive agenda.
Even now, if you go to either the Communist or Socialist Party websites you will see kudos to the RATs for their implementation of the progressive agenda.
Then there's the Progressive Caucus within the RAT caucus.