What was the makeup in 2006? Why does it appear that SUSA is weighing Democrats so heavily in their polls recently?
Because they aren't weighing for partisan ID at all. And because it is spur-of-the-moment self-identification they're asking, not registration (half the people lie and the other half don't remember on that question anyways.) If the country swings hard one direction... pollsters who do reweight lag behind developments.
Then again, not weighing (though they do weight for a lot of other factors like race, age, sex) also somewhat increases the risk of catching a wacky sample.
But as SUSA seems to be showing that in state after state right now... the "everybody else lags" argument does gain some credence... probably just a post-nomination Obama bump. Just like McCain looking like he actually had even odds at the presidency was just a post-nomination McCain bump.