Why Warren?

[Jim Lane]

21. Add Sheldon Whitehouse to the list

And, as others have mentioned, O'Malley and Schweitzer.

When you list "Brown" I assume you mean Sherrod Brown, and I agree, but it wouldn't completely surprise me to see Jerry Brown decide to give it another run. He has better progressive credentials now than he did last time. He's not perfect but he's much better than Andrew Cuomo.

As an aside, people are focused on Clinton, but I think there's a good chance she won't run. If she doesn't, Cuomo will, and he'll be a major threat. He's shown his "centrist" credentials by attacking unions, cutting social programs, resisting progressive taxation, etc., but he'll have something of a liberal image because he's from New York, he's a Cuomo, and he did take leadership on precisely one hot-button issue, namely marriage equality. If Clinton stays out, Cuomo will have the centrist image, the name recognition, the resume, the Wall Street backing (and the big financial advantage), and the support of the Democratic Party establishment. Any of the progressives mentioned will have a tough time beating him for the nomination.

As for your original question, saying "Warren" at this time is partly a reference to the senior Senator from Massachusetts and partly a metaphor for "generic progressive challenger" for Democrats who are tired of the corporatist policies. I wouldn't take it completely literally.