Presidential campaigns almost always advertise base off of something the place it is advertising to needs, even today. The sun in this situation shining behind the presidential candidate is a way of showing the candidate in a more hopeful, possibly even angelic view, like a gaurdian angel or a new leader and protector.
Any text where the main aim is to convey a point of view is a persuasive text and tries to convince a reader. An argument, exposition, debate, analysis or even an advertisement may be a convincing text.
