Adam Smith argues that political leaders rely on decorum to shape their rhetorical appeals, and that voters look for the fit between speech and character to gauge moral trustworthiness. In other words, the audience of a politician is attracted to appearance of sincerity in the politician's character. Insincerity of character can be politically disastrous. This is distinct from the politician's rhetoric being a good reflection of the audience's views and beliefs. Daniel Kapust and Michelle Schwarze argue that this is central to Adam Smith's account of propriety, especially in his Lectures on Rhetoric and Belles Lettres.
Smith illustrates his argument by criticizing Shaftesbury's use of ornamental language to cover up deficiencies of character. In other words, as Kapust and Schwarze summarize, "Shaftesbury's style was "pompous, grand and ornate," but his character and the substance of his works overall were not."
So, if Donald Trump is resonating with voters, Smith might infer this is because his over-the-top language actually reflects his true personality. It's not ornamental only for show, but effective at conveying authenticity. The more Trump's rhetoric is perceived to be a sincere reflection of his character, the more he can gain voter appeal. On the other side, perhaps Ted Cruz's failure to master a "Duchenne smile" leaves voters with the impression his decorum is off - his body language doesn't match his words and conveys insincerity.
But authenticity is the not only thing that matters! Smith warns that authenticity in politicians is often fragile.
"Cicero warns as Smith does that ornatus [flowery speech] is liable to abuse.... The problem a speaker faces is that "the very things that most stir our sense with pleasure and rouse them most strongly are also the quickest to give us feeling of aversion and satiety, and thus to alienate us" (3.98). Although ornament thus profoundly affects and moves its audience, it is a delicate and limited tool."
Kapust and Schwarze take the argument further, arguing that Smith explains how a shift in the political institutional context might determine the appropriate character and style of a political speaker.
"Smith provides a general framework for evaluating rhetoric (political or otherwise) by isolating three key variables that affect its reception: interest, the practicability of the thing recommended, and the honorableness of the subject (Smith 1983, ii.135).These three factors represent the standards, along with the orator's natural character, that constitute an audience's judgment of propriety, and in the same vein as Smith's stadial theory of history in both the Lectures on Jurisprudence and the Wealth of Nations, each is shaped by the historical and political environment in which one lives."
The authors then illustrate Smith's comparative institutional analysis of political rhetoric using examples from democratic Athens and republican Rome. In the former, the population was relatively politically equal but had come to rely on state support and transfers. Thus, politicians directed their rhetoric toward "..."one rank of Citizens," so that audiences evaluated speakers based on how successful they were at "coaxing them with new schemes of wealth" (Smith ii.159, ii.148)." In Rome, the lack of commerce led to the creation of static and unequal society. Here it was more politically advantageous to appeal to elite with the use of ornate language to signal hierarchy.
Hence, what works to rouse the passions and communicate authenticity is limited and shaped in important ways by the political economy context. The politician's rhetoric will have to cater to the interests of the electorate with practical promises of goodies and reflect some general notion of what they value. I read this as compatible with a median-voter model, populated with rationally irrational voters, where the candidates have to take positions closer to the center as they move from the primaries to the general election.