Sadr will take over Iraq?

I'm watching Hardball with Chris Matthews. He and his guests (one of them straight-talking Pat Buchanan) agreed that Muqtada al Sadr will take over Iraq in five years, after the US withdraws troops, and therefore the US presence in Iraq is a waste of time. Sadr take over Iraq? Yikes! I was surprised. I haven't watched Hardball in a while - how did they come up with Sadr to be most likely to lead Iraq in five years?