If you could have dinner with anyone dead or alive, who would you choose? You probably think I would choose somebody alive because a dead person isn't likely to contribute much to the conversation. However many living people are so dreadful that in many cases a dead person would make preferable company.
PUMAs, for those who have forgotten the 2008 Democratic primary race, were the supposedly-numerous Hillary Clinton supporters who refused to back Barack Obama. The name stood for "Party Unity My Ass!" which was also their rallying cry. This year, they may be replaced by the "Bernie Or Bust!" crowd, or (to coin a neologism) the BOBs.