In a nutshell: Only three Democratic incumbents have lost reelection in the last decade.
…which is interesting, until you remember that the Democrats lost seven Democrat-held Senate races in 2010, and one in 2012. If you’re wondering how that could be, well, either way you look at it it’s easier to maintain a high incumbent-reelection ratio when you’re sufficiently ruthless about getting weak incumbents out of the door. I don’t criticize the Democrats for that; pruning is what you have to do. But it does a disservice to your own party’s contributors when you pretend that any election cycle is like any other. The brutal truth is that this time around the Democrats have a large number of freshman Senators up for re-election who can’t be tossed out; and that their best two pickup states at this time are both long shots. So you assume defense, going in. And it’s an off-year, which will help the Republicans more.
The Republican party is in a good position, in other words. Not good enough to suit its own partisans, but a deep and abiding pessimism is frankly baked into that particular partisan cake and there’s not much that any of us can do about it.