The Reasoning Behind The Claim That "Make America Great Again" Is Racist

'Make America Great Again' Presupposes that America was better in the past
There was more racism in the past
Therefore:
'Make America Great Again' means (or is intended to mean) that racism is good

   I mean...this sort of thing is an idealization. People saying things like this aren't actually reasoning very explicitly.
  But, obviously, you might as well argue that "make America great again" means that we should have more heavy industry again, or more unions again, or less partisan hatred, or less urban violence, or more respect for tradition, or more family farms, or less cynicism... Saying that we used to be great but aren't anymore in no way specifies in which respects we allegedly used to be great.
   In fact, of course, it means: We were awesome and Obama and the Dems screwed everything up and now we're not awesome anymore but I, Trump, can make us awesome again. It's basically boilerplate political bullshit with a conservative and anti-incumbent twist.
   If we get to just pick out some nefarious meaning for campaign slogans, then we also get to say...oh, hell, whatever... Hillary's "stronger together" means that we'd be better off ceding more power to the UN...or that "fighting for us" means fighting for the Democrats. Once you allow this kind of crap, everything is fair game, nothing is too stupid.
   Another possible way to represent the reasoning that got this started is:

Trump is racist
Trump says we should make America great again
Therefore"
He must mean something racist by it.

   Perhaps more likely, it's a combination of things. Trump has a real and undeniable inclination to say things that are consistent with bigotry. And he says a fair number of such things. So the bigotry hypothesis really does present itself pretty forcefully. But in addition to that, the extremist PC/pomo left is currently extremely influential, and the PC/pomo left has a tendency to employ a method of literary free-association. And they've dragged the left edge of liberalism in that direction. (Or the left edge of liberalism went that way willingly...) If they can think of a possible interpretation of p that serves their interests, then they tend to declare that interpretation to be the meaning of p. They're driven by identity politics, so they're mainly focused on finding possible interpretations that indicate (alleged) prejudice. This + Trump: charges of racism aplenty.
   But this nonsense about "make America great again" being racist is, IMHO, by far mostly a product of the lefty imagination. It's mostly about confirmation bias. And this is a general problem on the left, the tendency to accuse the right of racism on shitty grounds at every opportunity. And that's one of the shittiest things about contemporary liberalism. And if liberals don't care that they are shitty, and that they have an obligation not to be shitty, then maybe at least they'll care that their shittiness makes conservatives stronger and fans the flames of Trumpism. Which, of course, is not something that you should focus on when you are shitty...
   And, dammit, liberalism needs to stop leaning on the fact that conservatism is currently shittier than liberalism. It's leaning on that excuse that's made liberalism shittier and shittier, until the shittiness gap is way narrower than it used to be. That's an almost inevitable result of relying on that excuse.
   And get off my damn lawn while you're at it.