Why do I think the Republicans will do better than expected? Because a pair of twos beats an ace high. Sure, the Republicans have made lots of mistakes and have given people in a lot of districts reasons to vote against them (scandals and the like.) But Democrats have not given people a corresponding reason to vote for them.
Part of the main message from the national Democrats has been "change is coming", counting on the dissatisfaction people have with the status quo. But "change" messages only resonate when people are dissatisfied and restless. The American people are dissatisfied, all right, but I think their secondary attribute is vagueness: they don't like what's happening, but they don't quite know what to do about it. Vagueness doesn't drive a desire to sign up for a GOTV drive, and it really doesn't help turnout.
All the conventional wisdom points the other way, I know. But the thing about the term "conventional wisdom": it's half right.