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.
I don't really see why it's wrong that Nate has political opinions. The alternative is Jim Lehrer who values his "objectivity" so much that he doesn't even vote at elections. Sometimes, facts favour liberals, sometimes they don't.
The overrating of objectivity has been a hobbyhorse of Jack Shafer's for awhile, and he can be fun to read; here, for instance:
http://blogs.reuters.com/jackshafer/2011/09/20/media-bias-gi...