It's worth noting -- and remember this when people tell you Dems dropped the ball in the Senate should they lose -- that since they won back the majority in 2006, Senate Democrats have consistently exceeded expectations. They won not only nearly everywhere they were supposed to, but in places where they messed up (i.e. Pennsylvania) they made up for in spades in Indiana, North Dakota and Missouri. Scott Brown had a short-lived period of bragging rights for having Ted Kennedy's seat but Elizabeth Warren took care of that.
To put it simply, they overperformed. I'm surprised they were able to hold on to the Senate in 2012 quite frankly.
The only Democrat who fucked up big time is Bruce Braley, who talked shit on the voting populace in a hidden video that severely hurt the party's chances in Iowa this year. That was a huge, huge mistake, and from 2006-2014 is honestly the one single thing I can think of that Democrats REALLY got wrong when campaigning for the Senate.
And even with all of that being said, this year heavily favors Republicans by default because of a. the seats in play and what states they are in and b. how that relates to turnout in a midterm election. This isn't even factoring in Obama's approval rating, Ebola, Isis, the economy, etc. yet. It's just a bad map for Democrats, period.
Democrats have basically done everything they could possibly do to hold the Senate, outlasting my initial prediction that they'd lose it no matter what in 2012 (even with Obama re-elected). Depending on how things go, Braley might end up being the guy who keeps it from being 50-50 or 51-49, and if that's the case that would be a shame. That said, if he loses there's a good chance Begich, Landrieu, Hagan etc. could lose too so it won't even matter (which may very well happen).