Earlier quoted context omitted.
I don't think he has a leaning one way or the other - he tries to be unbiased, and arrive at data-driven conclusions. What have you seen from Nate that makes him seem a "biased liberal" to you? I'd be very interested to know. Also, we'll see in about a month how good his meta-analysis is.
He leans Democratic and has said so on more than one occasion. He supported Obama in '08. One gets the impression that his professional reputation is more important than his ideological affiliation, and so I find him pretty credible.
"What is your political affiliation? My state has non-partisan registration, so I am not registered as anything. I vote for Democratic candidates the majority of the time (though by no means always). This year, I have been a supporter of Barack Obama."
And the next question -
"Are your results biased toward your preferred candidates? I hope not, but that is for you to decide. I have tried to disclose as much about my methodology as possible."