It's the reverse of the question for conservatives: which Senate candidates do you really care about? Two parts...of the endangered Democratic incumbents, and ignoring strategic questions and anything about the opponents, which ones do you care most about saving? Which ones would you not mind losing? I'll leave Lincoln out of this (since it's a question for liberals)...the other endangered incumbents are Boxer, Murray, Feingold, Reid, and Bennet.
OK, how about the challengers and open seat candidates? Again, put aside anything other than which ones you think would make great Senators. The Democrats in contested races are Blumenthal, Manchin, Coons, Giannoulias, McAdams, Meek (or Crist?), Ellsworth, Carnahan, Conway, Hodes, Fisher, Sestak, Marshall, and Melcanon. If you could pick three to be Senators, which ones? If three have to be left behind (and, for Democrats, that's of course a very optimistic scenario, but...), which three?