The present paper explores the synchronic variations and diachronic changes in political discourses in Hong Kong (HK) and in Mainland of People’s Republic of China (PRC). The relationship between lengths of linguistic constructs and their immediate constituents (including sentences and clauses, and clauses and words) are fitted using the function y = axb based on the Menzerath–Altmann (MA) law to capture the characteristics of language as self-organizing complex systems. We found that the two fitted parameters a and b, as distinctive characteristics of complex systems, can distinguish two regional variants of political speeches from HK and PRC over different periods in time. We also found that the same parameters can capture language changes between different periods of political speeches from the PRC. More specifically, we found that regional variations and historical changes show different degrees of salience at different constituency levels. In addition, we found compounding effects between historical change and regional variations. That is, the two regional variants of political speeches are closer to each other at the earliest diachronic period as compared with the latter two periods, as represented by the fitted parameters of the relationship between sentence and clause lengths. Our results provide strong support for the hypothesis for the MA Law capturing the characteristics of language as a complex self-organizing system, as the two fitted parameters account for the interaction of diachronic language change and synchronic variation.