I have another comment in this thread that lays out how this falls in line with the use of federal power to get the states/cities to act in a certain way without declaring an insurrection. Beyond that, I'm not sure if it does clearly help the President (or hurt him) overall.

His base will see it as him striking at the core of the Democrat interests in a way that will either result in driving more people out of those enclaves (weakening Dems) or forcing the people to change up the leadership in their cities (also weakening Dems). His opposition will see it as an abuse of power that has already been repudiated by federal courts (see defunding sanctuary cities still making its way through appeals) and proof that he is a dictator that wields his powers to enrich himself. The fence sitters will either fall on one side or the other or will ignore the whole thing.

It could also be that this isn't a political move aimed at gaining votes, but is an effort to break the current logjam. Consider that at this point, everyone seems to be in agreement that the continued riots and looting is and will help Trump. It makes political sense then for him to offer federal help so the blame is on the mayors and governors, but when they do ask for help it is provided right away. It can be argued that the President is really trying to get law and order re-established as quick as possible without expanding or overstepping the bounds on the federal government, even if not doing so would help him.

It could also be argued that the above paragraph is exactly why he would do it because it would make him look good.

Either way, if this goes through, the President will be blamed for the bankruptcy of the cities by those who oppose him. His supporters will blame the cities and those that lead them for the same thing. Hopefully, everyone is hoping that the defunding doesn't go through because the cities come back under control and the violence stops.