Why don't US parties have opposition 'leaders' [View All]

[Astrad]

I'm not all that up on the american political system so excuse me for this question. But in parliamentry systems, opposition parties (parties out of power) have leaders who exercise tremendous control of policy and issue positions. It doesn't seem to do a lot of good for the Democratic party not to have a leader. It seems to emphasize the notion that there is a lack of cohesion in the party. Why not elect a leader now who will run for president in 2008 and who exercises some discipline over the party while in opposition?