Final answer:
The rhetorical device present in the provided lines is Parallelism, which involves a repetition of the same grammatical structure to elevate the importance of the ideas expressed.
Step-by-step explanation:
The rhetorical device used in the lines "But we have always understood that when times change, so must we; that fidelity to our founding principles requires new responses to new challenges; that preserving our individual freedoms ultimately requires collective action." is Parallelism. This device involves using the same pattern of words to show that two or more ideas have the same level of importance. The construction of these phrases demonstrates parallel structure, as it repeats the same grammatical form: 'that ... requires ...; that ... requires ...; that ... requires ...' This repetition of the structure enhances the rhythm of the speech and makes the message more memorable and impactful.arallelism is the repetition of grammatical structures or patterns for emphasis and clarity. In this case, the repetition of the phrase 'that' at the beginning of each clause creates parallel structure.