Final answer:
The behavior of the former senator is an example of the plain folks theme, used to make the candidate appear approachable and connected with the average voter.
Step-by-step explanation:
The behavior of the former senator dressing in denim jeans, a down vest, and a baseball cap at steelworkers union rallies before announcing his presidential bid is an example of the plain folks theme. This approach is commonly used in political campaigns to create an image of the candidate as being of the people, approachable, and one with the everyday citizen. Instead of appearing as a distant political figure, the candidate aims to connect with voters on a personal level by embodying the lifestyle and preferences of the average person. This tactic is contrasted with others such as the bandwagon theme, where winning the support of the general population is crucial, or the glittering generality, in which broadly appealing, but vague, statements are used to win favor.