Jay Cost with the Weekly Standard had this to say about the Pew poll sample:
Is it really THAT implausible? Given that a lot of GOPers are old, there's a decent chance that a fair chunk that voted for McCain in 2008 have since died. I guess of course that 2008 was an anomaly because of Obama's cross-over appeal. But I would think turnout numbers like 2008 are going to become the rule rather than the exception, as more old Republicans pass away.
BTW Jay Cost is a huuuge asshat.
Following that reasoning the GOP will just disappear as its supporters die off, I highly doubt that will happen, but I do agree that the 2008 figure is an anomaly. Republican turnout will more likely than not be around the 35% mark this time around.