Final answer:
The priority action by the nurse is to maintain adequate hydration.
Step-by-step explanation:
The priority action by the nurse when planning care for a newly admitted patient with bipolar disorder and experiencing mania is to maintain adequate hydration. During manic episodes, individuals with bipolar disorder may engage in excessive physical activity and neglect to eat or drink properly, which can lead to dehydration. Ensuring that the patient is properly hydrated is crucial for their overall well-being and can help prevent further complications.