In reality he is in part.
Someone said here... "Humans tend to "mirror" one another's behavior.", and there is a lot of true to that. Humans have this thing where they start to follow trends set by other more prominent humans.
This is mostly because a good portion of people -- especially in America -- are complete morons who are just swayed back and forth by the things they see and read, and also by their affiliations and social circles.
For example, when Trump started to praise Putin early in this administration, many republicans started to replicate the things he was saying. Imagine that, republicans praising a pseudo-communist dictator. How is this possible? Well because they are following in line with that trend.
The POTUS does set the temperature on many issues, especially for the droves of morons and idiots that walk the streets of this country, who don't have the ability to discern things for themselves