There's not really any question that zombie movies reflect left-leaning politics. Still, this article by the The American Prospect's Paul Waldman leaves something to be desired:
But most important, what ensures survival in a zombie story are the progressive ideals of common cause and collective action. A small group of people from varying backgrounds are thrust together and find that they can transcend their differences of age, race, and gender (the typical band of survivors is a veritable United Nations of cultural diversity). They come to understand that if they're going to get out of this with their brains kept securely housed in their skulls and not travelling down some zombie's gullet, they've got to act as though they're all in it together. Surviving the tide of zombies requires community and mutual responsibility. What could be more progressive than that?
But the survivors rarely act as if they're "all in it together," as George Romero's movies make abundantly clear. The humans always turn on each other. By Land of the Dead, Romero was basically rooting for the zombies.
For a far more sophisticated and convincing take on the mostly left-wing politics of zombie cinema, look no further than Tim Cavanaugh's Reason classic "We the Living Dead":
From Night of the Living Dead to Homecoming (in which dead Iraq war veterans return from the grave to vote against the war), the zombie movie has been among the most consistently political forms in American popular culture. The politics tend to lean left, but zombie entertainment approaches a level of discontent more elemental than mere anti-capitalism or shopping mall burlesque. Apocalyptic and piously disdainful of the carnal realities of human life, zombie cinema is a shocking, uproarious meditation on the nature of death—on what, if anything, we owe to the dead.