Yeah, he could have definitely bumped his approvals up to at least 47-48% if he campaigned and Democrats supported him. When voters hear "Obama is the worst president ever" from the Republican candidate and "meh, maybe, I don't know" from the Democrat candidate, what are they going to think?
Also I'm 100% certain that Braley would have lost even if the environment were like 2012. He was an awful candidate. Udall will end up losing by only 2% against one of the best Republican senate candidates in a long time.
My guess is Udall got so close because of CO Dems still have a very good turnout machine.