Be sure to check the link on Baucus' co-op plan. The last paragraph explains why a state or regional co-op just won't cut it.
Presumably, Conrad’s co-ops would act more like health care plans and less like health insurance exchanges. Unlike the traditional co-op which strives to give its members a choice of plans, Conrad’s co-op might either self-insure or contract out to a third-party administrator. But state-based or regional co-op health plans would be unable to exert the purchasing power of a Medicare-like public option. Whereas a public health care plan could use Medicare’s leverage and Medicare-like prices to negotiate lower prices and — through the miracle of head-to-head competition with private plans — push insurance companies to negotiate more aggressively with providers and dramatically lower health care spending, a cooperative will likely lack the clout to demand lower prices.
http://wonkroom.thinkprogress.org/2009/06/19/tri-committee-bill/