The debate over an overhaul of the nation's healthcare system got off to a rocky start in the Senate as lawmakers delayed action on one key bill and engaged in partisan sniping over another. Senate leaders said they are still on track to put a bill on the floor by midsummer, but some Democrats privately acknowledged that piecing together a measure that will expand coverage to the uninsured is proving excruciatingly difficult, particularly if the goal is to pass a bill with Republican support.