There's a liberal bias. The networks have a bias. Especially ABC, and CBS. The major newspapers like the New York Times, Washington Post, and Los Angeles Times are liberally biased.
If you honestly wanna say with talk radio, and FAUX NEWS it's in the middle, I'd listen. But no chance it leans right.
Alterman probably does think it leans right, because it's not far left enough for his tastes.