If voters are stupid enough to actually elect Gardner, that's not Udall's fault. Sometimes, voters are determined to elect the worst possible candidates to office.
No. Dem incumbents with a good record in a lean-D State in a neutral year don't normally lose reelection. Udall screwed up, there's no other way to put it.
Colorado has only very recently become lean Dem, and it's swung back and forth before. This is the kind of seat Dems should expect to lose in a decent Republican year which is unfortunately what 2014 is starting to look like.
Dem incumbents in more diverse swing states (VA, NC, even LA all things considered as a non-swing state) are holding up much better than Dems with much less diverse electorates (CO, IA) and for that reason I'm seeing Scott Brown as a surprise winner now.