What is the ideological basis for: 1) parties like PSL running candidates and 2) parties like SAlt, CPUSA, and others supporting Democratic candidates?

I didn't post this topic in com101 because I feel like this request for theoretical documentation is a 102 question. I know this might be a beaten to death topic but I've never actually seen the ideological basis that communist (or "communist") parties use to justify things like running in bourgeois elections and supporting bourgeois candidates in bourgeois elections. So what theoretical documents or books do they use to justify these things? (And in the case of Socialist Alternative, I'd like to learn what aspect of Trotskyism leads to this, so I'd prefer if someone cited sources on it.) via /r/communism https://ift.tt/2PwZolJ