Well, I don't exactly agree with the position you have to take, but obviously that isn't the point. You just have to make the argument for the sake of debate & to stimulate thought on this issue!
First of all, don't take it too seriously. Just think about what that side typically argues:
Begin by clarifying what is MOST important in nursing care: PATIENT SAFETY. A nurse is the main coordinator of pt care. The nurse has the training, education, and license to provide safe care. In order to obtain a license, a nurse must go through a rigorous education program, clinicals, peer assessment (ie. RN instructors/clinical preceptors judge the student's ability to provide care), and a formalized testing process. Nurses are trained to understand medical issues/terminology & to work with a variety of teammembers (ie. PTs, OTs, MSWs, STs, MDs, etc...). The MDs do the ultimate decision-making regarding a pt's diagnosis & the type of treatments pts need, but RNs are responsible for understanding enough about the MD's decisions to carry out the medical plan of care. NAs are not trained to do this. Not only are RNs trained to do this, but they are also legally responsible (due to licensure) for carrying out only those MD orders that make sense -- in other words, if an MD decision is irrational or unclear, an RN is legally responsible for calling the MD, asking for clarification, and then determining whether the order really makes sense. NAs do not have this training.
Nurses manage the pt care from admission to discharge. Not only do we carry out MD orders, we also: make sure that all the ordered treatments/tests are run (ex. MRI, CT scan, blood work, pulmonary function tests, etc...), the correct diets are ordered & given to the pt, that PT/OT have actually seen the pt as ordered, that the MSW has arranged discharge destination/transportation/payment for hospital services, that all of the family's questions are answered regarding the pt's diagnosis (we chase after docs & ask them to TALK to the family/pt), monitor the status of the pt & his/her RESPONSE to treatment, ensure consent forms have been signed, etc... We assess all of the issues facing the pt, and help them problem-solve. Truly, the nurse is the pt's primary advocate, for we ensure that they receive the optimal care while hospitalized. In essence, nurses have a holistic understanding of pt care -- we understand what that pt is having done from every discipline, and we make sure that it is actually carried out. We have an overall view of pt care that is not shared by the CNA. Therefore, the RN has a broader view & higher vantage point than the CNA regarding the care of an individual pt. This means that the RN would have to delegate to the CNA if vital signs needed to be taken more frequently, specific vitals would have to be more closely watched (ex. O2 sats for asthmatic pt, bp for certain medications), I's/O's needed to be strictly measured, a pt's neuro status would have to be more strictly observed, etc... The RN has more in-depth KNOWLEDGE about what the pt needs in the plan of care & the RN is better trained to make DECISIONS regarding this plan.
In addition to the medical plan of care, there is an interrelated NURSING plan of care. Nurses are really responsible for basic comfort measures & the overall well-being of the pt while hospitalized. Nurses are ultimately responsible for (& LEGALLY licensed to treat) those things described by nursing diagnoses: skin care (preventing pressure ulcers by turning pts at least q2h if bedridden, doing skin assessments, calling MD or woundcare RN if pressure ulcer), toileting (I's/O's, preventing constipation, skin care r/t), aeration (ox sats monitored, must protect airway, call MD is O2 indicated or some other respiratory tx, monitor RR & lung fields), observing family dynamics (dysfunction, support systems), listening to the expressed (and unexpressed) psychosocial needs of the pt, etc... The nurse is responsible for the care, but the CNA's function is to help the RN carry out the NURSING plan of care -- in essence, the RN is LEGALLY RESPONSIBLE for what the CNA does since the RN is legally responsible for carrying out the nursing plan of care. In my state the RN is even legally responsible for & authorized to delegate work to the CNA.
IMPORTANT: Something you should emphasize in your argument, since it should be included in any discussion about delegation & the RN/CNA working relationship: The CNA is the "right-hand man/woman" of the RN, for they are invaluable to effective/competent nursing care for the pt. I respect, admire & heavily rely upon the CNAs on my unit. I think we have a great working relationship, and pt's are greatly impacted by the compassion & sensitivity demonstrated by CNAs (in fact, sometimes it is the CNA that most dramatically impacts a pt's stay on the unit, for all humans need respect, touch, compassion and support during times of stress/illness -- often the CNA functions most effectively in that role for a given pt in a given circumstance in a given point in time).
Finally, address the counter-argument that you know your debating opponents will make. They will argue: there are certain functions that can be broadly applied to all pts, therefore implaying that one standardized, defined role description is sufficient for CNA training/function (ex. empty foleys, turn pts, take vitals). However, this does not account for circumstances where a pt does not FIT a standardized plan of care -- since MOST pts are unique, the monitoring they need is unique -- since a nurse has a better understanding of the nursing/medical needs of a pt, a nurse will have to delegate to the CNA those tasks relevant to their individualized plan of care. The RN needs information specific to the pt's individual plan of care. This requires delegation to the CNA in order to adjust info-gathering (ex. the RN tells the CNA that vitals must be obtained q2h rather than q8h since the pt is very unstable). The CNA may collect info (such as vitals), but only the RN (as opposed to the CNA) can legally interpret the info & act upon it.
CNAs (or other unlicensed personnel -- a certification is NOT licensure, by the way) operate under a "defined role description", but this role could be interpreted as "assisting the RN in performing nursing care". This implies that delegation is REQUIRED, that it is inherent in the definition of an unlicensed personnel's job description, for only the RN is legally responsible, competent/trained to determine the nursing plan of care. The RN is the ultimate decision-maker for the nursing plan of care, just as the MD is the ultimate decision-maker for the medical plan of care. The CNA's role does not EXIST outside of the nursing role. Therefore, they are not independent in decision-making of pt care, unlike the RN or other licensed personnel.
Does this help?