538 is pretty open, regardless of the political beliefs of its creator.
It's like Football Outsider's DVOA. It takes in stats and spits them out - the creator's favorite team doesn't play a part.
If you believe there's a calculation that's giving a "leftist" interpretation, please show us where. The methodology is open for critique:
Also, where were the arguments about a leftist lean when we got clobbered by the Tea Party nationalists in 2010?