If he pulls out of Afghanistan the Taliban will probably defeat the inept Afghan government we've been propping up all these years and celebrate the defeat of the "Great Satan" in the streets of Kabul. Also celebrating behind closed doors will be the Rethuglicans who will ride this result all the way back from the political purgatory chimpy put them in. Can you imagine how loudly they will howl that Obama is a weakling on national security who doesn't have the courage to stand up to America's enemies?
Ironically, it would be much safer politically for Obama to continue our useless effusion of blood and treasure in Afghanistan. It would take REAL COURAGE to stand before the American people and explain why it makes more sense to pull out and acknowledge that all the expense and sacrifice of the last 8 years has been in vain. It would take REAL POLITICAL SKILL to do this without comitting political suicide in terms of the political fortunes of himself and his party.
This article by Dan Froomkin in the Huffington Post does a pretty good job of laying out Obama's dilemma:
http://www.huffingtonpost.com/2009/11/19/obamas-afghan-dilemma-the_n_363411.html