The Allen event that seems most similar to the recent McDonnell revelations came several months before the Macaca incident that everyone remembers. In April of 2006, TNR's Ryan Lizza came out with a big article on George Allen's love for the Confederate flag, which included the revelation that Allen had been wearing a Confederate flag pin in his high school yearbook photo. With that, and a bunch of similar things in the background suggesting that Allen wanted to support white people's struggle against people of color, the Macaca scandal had exactly the background it needed to damage his efforts for re-election.Context matters, and so the things that establish context may turn out to be very important even if they have little immediate impact.
I bring this up because it answers the question that Kevin Drum is asking about the school speech flap: "This stuff just has to backfire on them eventually, doesn't it? Please tell me yes." Yes, Kevin, it does. It's just that it doesn't always backfire immediately (although sometimes it does) Instead, each time the Republicans embrace the crazy, it makes it that much harder for honest people to stick with them. You can never tell what will push people over the edge -- will it be death panels? Hitler signs? But while all of us expect our side to do dumb things sometimes, and all of us understand that in politics our side is going to be driven to short-term demagoguing sometime, most of us have some limits, and each violation makes it a little more likely that we'll turn away for good when our side does something that strikes us as really awful. The same thing works for the press and for Beltway reputations; they aren't created or destroyed based on one episode, but built up (or down) over time.
The more obvious answer to Kevin Drum's question is, of course, that it backfired long ago. That's why the Democrats have picked up some fifteen Senate seats, seventy House seats, and the presidency over the last two election cycles (along with plenty of gains in statehouses and state legislatures, too).