In a discussion of the rhetorical styles of Caesar and the early principes, Fronto formulates the maxim that imperium…non potestatis tantummodo uocabulum, sed etiam orationis (‘’command’…is a word connoting not only power, but also oratory’ [p.123.16-17 van den Hout]). This essay will explore the political background and implications of trends and shifts in Roman ways of thinking about language and oratory in the transition from Republic to Principate. The word declension in my title functions in two senses: literally, in the case of Caesar's discussion of the nature of the Latin language (in De Analogia) and his rivalry with Cicero's views on oratorical style; and figuratively, in the perception of decline in oratory expressed by the elder Seneca and other writers of the early Principate. I hope to be able to present a new approach to, and understanding of, both these aspects.