I agree Silver is relatively moderate, but FiveThirtyEight did more or less grow out of DailyKos [1], so it's not a huge stretch to suggest he personally has a clear preference for one or the other side of the American political spectrum. It's not like he also had a blog on the right-wing equivalents of DailyKos, like FreeRepublic or RedState.

[1] For a year or so before he launched fivethirtyeight.com as an independent site, he posted data-driven election analysis, among other things, at http://www.dailykos.com/blog/poblano