This is my purely mercenary analysis of the situation.
As WC Fields once said, "I never vote for anyone; I always vote against." While hardcore voters won't be swayed from the straight party ticket, it's the "WC Fields" vote that wins elections.
Normally, the Democrats are an easy target to oppose. The problem for Republicans is that their performance to date has been so abysmal that most people want to vote against Democrats and Republicans, in equal measure. They want what the Republicans could bring, but don't, and that's a hard sell when you're an incumbent defending your seat. "But my opponent would be worse" is normally a pretty effective argument, but you can negate that advantage by your own incompetence.
Candidates can't just point the alternative is worse. You have to make them feel it. Voters have to feel less disgust when looking at a Republican than a Democrat, in order to pull the lever for them. Right now, most voters aren't feeling it.
Of course, insulting said voters, on top of failing to deliver for them, won't make you any yardage. Calling people stupid for not voting a certain way is another step towards a well earned defeat.