Final answer:
John F. Kennedy possessed both rational-legal authority as the elected President and charismatic authority due to his personality and the promise of progress under his 'New Frontier'.
Step-by-step explanation:
John F. Kennedy's leadership was legitimated by rational-legal authority and charismatic authority. Kennedy's charismatic authority came from his appealing personality, youthfulness, and the optimism he inspired in the American public. His vigorous campaign and the promise of the 'New Frontier' resonated with a nation eager for progress and change.
Kennedy's ability to connect with diverse groups of Americans—including labor unions, minority voters, and young people—and his reputation as a war hero further solidified his charismatic appeal. Though he faced challenges such as concerns over his inexperience and his Roman Catholic background, Kennedy's charisma, combined with his legal authority as the elected President, allowed him to possess an enigmatic leadership style that captured the hearts of many and became emblematic of the era.