Yglesias explains why the US needs Karzai to win:
For Abdullah to win, he’d basically need the Taliban to successfully scare enough Pashto into not voting to overcome his intrinsic disadvantages. That would, in turn, be a boon to the Taliban war effort. Karzai’s government has long suffered from the perception that he’s just a Pashto face on a US/Turkic regime but if you turn that into a Tajik face that only won thanks to Pashto non-participation you’ll be looking at a real disaster.
Realistically for Afghanistan to be governed successfully you need a Pashto at the top working in alliance with a few key Tajik and Uzbek power-players. The talk about election processes and recounts seems to be to obscure the basic real shape of semi-feudal Afghan politics. The real legitimacy issue, meanwhile, probably will be decided less by national elections and more by practical results on the ground—are the people running your town honest and competent?
At any rate, this is all sort of too bad. I’m a long-time Abdullah fan since back pre-9/11 when he was the only senior member of the Northern Alliance who could speak English and you see him quoted on A-7 New York Times stories about Afghanistan. And his platform (PDF) calls for Afghanistan to abandon its presidential system in favor of a parliamentary one which is absolutely correct and also exactly the kind of thing I like to go on-and-on about.