So politely? Detroit doesn't need you to defend it, nor is it a big own to say "Detroiters will remember this at voting time" - Detroiters were never gonna vote for the fascist regardless.
If you want to help, then start talking about the motivations and ideology behind WHY Trump talks about Detroit the way he does - and those motivations and ideology are white supremacist, and fascist in nature. If you can't be arsed to SAY THAT, then just shut the fuck up because you ain't helping.