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.
I still think Udall will win, but if he doesn't, I really don't see what else he could have possibly done to win this race.
Campaign on more than one issue. If he really wanted to win this, he should have both bragged about his overall record in the senate and attacked Gardner on multiple issues. Instead, he assumed 'Gardner = Buck' (which is wrong on so many levels) and simply spent the last several months saying things along the lines of "I support Abortion, Gardner doesn't" over and over again, hoping it would work.