There has considerable argument over this.  Both Clinton Nad Obama's plan are very similar.  Basically, they allow for entry to the Federal Employee Health Care plan for those who can't afford health care insurance.  Clinton mandates coverage while Obama leaves it as optional.

I see this as potentially a back-door regulated multi-payer system.  If the Federal Pool is opened to everyone then we would all be in a highly regulated insurance pool.  It would be impossible to deny coverage.

This sort of model is common overseas; perhaps, even more common than single payer.  The disadvantage is that you still have the "middle man" though it would be kept under control.