This phrase is a code phrase; that is, it can mean different things to different people. Most conservatives see it as a call to bring back from America's past, small less obtrusive government with less federal spending, banning abortion. or less illegal immigration which have all existed throughout most of American history. However, to extremists it can mean putting black people in their place before the civil rights movement, closeting homosexuals and other sexual deviates, and cleaning American of illegals and foreigners.
Campaign slogans usually fall in 3 categories:
Those that are capitalizing on the popularly of the candidate - "I Like Ike"
Those that focus on a major issue with voters, "No New Taxes. Read My Lips."
When the issues are more popular than the candidate, a phase such as "Make America Great Again" is appropriate because it mean whatever a potential supporter wants it to mean.