Obviously, Trump's aim is to cut deep into the black vote, by laying the blame for the decayed urban cities many are forced to live in, on Democrats and forcing them to defend that record. Blacks are Democrats' political base, and they're also the prime victims of blue-city policies as well as all the neglect inherent in them as Democrats focus on saving the earth by banning plastic straws or else offering free health care to every illegal border crosser, so it's an unusually bold attack.
How are Democrats going to defend themselves from this one? In one-party blue cities, states and districts, there's no one to blame but themselves. Trump's tactic is particularly ripe for propagation in solid-blue, shambling expensive California and already Trump is moving his attack onto Pelosi with his latest tweets.
Thus far, Democrats are fighting back by yelling 'racism.' It's a tired, weak weapon, grotesquely overused, and unlikely to rally black voters to Democrats, given the truth of Trump's tweets. What's more, it can't be employed by Pelosi, who's white. The issue and the narrative Trump is pushing out there is that all-blue cities are hellholes and they've been made hellholes because Democrats did it.