Sunday Question for Liberals

Same question that I asked of conservatives. With the debates winding down for this cycle, what lessons if any should Democrats learn for 2016 about the way they were organized? What worked? What didn't?

Of course, the incentives of individual candidates and of the potential sponsors of debates may well differ from those of the party as a whole. But to the extent that the party could change things, what should be changed and what should be retained?