Pomeroy was in perpetual danger until 2004 - interesting you post results after that. 
I guess he's the strongest challenger, though I really don't know about other possible Dems - they control a couple of statewide offices, right BRTD? (He's probably our ND expert - especially among Dems).
Only two: Agricultural Commission Roger Johnson and School Superintendent Wayne Sanstead, and the latter is elected on a non-partisan ballot (North Dakota for some reason has an insane number of elected statewide offices, including three elected Public Service Commissioners elected to a 6-year term, so there's one up every two years.) Both have been in office forever though, I remember campaign ads for them in elementary school. Either one is more likely to go for the House seat though, Pomeroy will probably end up the anointed candidate and would most likely beat anyone except Hoeven, the only other candidate I could see posing a threat to him is Attorney General Wayne Stenehjem. In that case though the House seat has to be seen as at least Lean R, unless Johnson or Sanstead run (neither is guaranteed, the only other viable candidate is former Attorney General Heidi Heitkamp) and the GOP nominate someone really bad/have a bloody primary (both likely.)
Actually a bloody primary is pretty much guaranteed unless Hoeven runs, so that should work in the Dems' favor at least.
Johnson isn't in office any more, he resigned to head the National Farmers Union.
