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At the risk of embarrassing myself badly if someone points out an obvious mistake, I’ve stitched together an interesting pair of charts for you to look at tonight. But first some background. As you may know, the latest Gallup generic poll shows a sudden surge of support for Democrats. Why? Gallup suggests it might be related to passage of the financial reform bill, but I’m skeptical of that. Democrats have gone from 2 percentage points down to 6 percentage points up in only two weeks, by far their biggest jump in the past four months. Could a complex and barely understood regulatory bill really have caused that?

Maybe, but here’s another possibility. It turns out that the Democratic surge is largely due to a sudden jump in support from independents. So what caused that? Well, I was struck by an unusual correspondence between two of Gallup’s charts. It turns out that whenever enthusiasm goes up among registered Republicans, preference for Republicans goes down among independents. The pasted-together chart below — it’s a little messy I’m afraid — shows five cases of a jump in Republican enthusiasm (top chart) along with the corresponding drop in Republican support among independents (bottom chart). It’s not a perfect correlation, but it’s a pretty good one.

Anyway. Here’s my guess: every time Republicans do something that gets the tea party base excited, it simultaneously turns off independents. I’m not quite sure what caused the latest jump (NBPP fever? tax cuts pay for themselves? unemployment compensation obstructionism?), but apparently it was something.

So this is the GOP’s big problem for November: they need to motivate their base, but their base is so stone crazy that the only way to pander to them is with tactics so outrageous that non-crazies start to turn away. So far this hasn’t hurt them too badly because the independents tend to come back until a fresh provocation hits the airwaves a few weeks later, but eventually this might catch up to them. There’s obviously no rigorous statistics involved here, just sort of a gut feel. Take it for what it’s worth.