"Make America great again."
It's got a nice ring to it, doesn't it? Regardless of how you feel about "The Donald," you've got to admit it's catchy. When it comes to American politics, don't underestimate the power of a good campaign slogan! (Remember "Yes we can!"? Whatever that means to you, it ended up meaning a win for Obama in 2008.)
The other aspect of a good campaign slogan is that it can mean whatever you want it to mean, making it feel personal to a wide range of people. If you've been fed up with X,Y, or Z recently, you may feel that "making American great again" will solve your problems. While "Yes we can," ambiguous as it was, offered a sense of hope to the disenfranchised.
The wording is very important here. For example, simply "Make America Great" doesn't have nearly the same ring. Why not? Because it wouldn't appeal to the dissatisfied masses who feel that something needs to change. (Almost regardless of whether that change is good or bad, but that's a topic for another day.) It also appeals to the nostalgia we all have for some shining past moment of glory when our country was (presumably) greater than it is today.
However, genius as this slogan is, I have a couple of problems with it.