Final answer:
The best-completing sentence for the diagram is that political attack ads have played a significant role in election campaigns and influenced voter opinions. The evolution of these ads from Lyndon Johnson's era to the present involves substantial investment by super PACs and a potential future decline in their effectiveness.
Step-by-step explanation:
The sentence that best completes the diagram concerning political attack ads and their impact might be: Political attack ads have evolved to become a significant component of election campaigns, often swaying the opinion of voters and sometimes impacting the results. Since the 1964 election and the impactful 'Daisy Girl' ad by Lyndon Johnson's campaign, the usage of these ads has proliferated. Campaigns, political action committees, and super PACs have consistently utilized attack ads to influence voters, from Nixon's era to the 2020 elections where super PACs invested billions.
Despite their prevalence, some scholars argue that the effectiveness of negative campaign ads may wane due to new social media dynamics. Additionally, while ads like 'The Daisy Girl' have been instrumental in the past, strategies are ever-evolving with changing voter behavior and attitudes towards such content-loaded campaign tactics.