I honestly don't remember, but I was probably trying to point toward the Wikipedia stats, in which case I shouldn't have used "registered". A quick search for registration percentages turns up this, which cites slightly under 60% registration in the most recent election (it's been going slowly down over time; was apparently just over 70% in the late Sixties). I haven't been able to turn up party-specific registration figures; I suspect but cannot prove that you're underestimating the number of Americans registered as independent.