There is a harm, of course. When more people vote, the value of my vote is reduced. So is everyone else's.
What happened here is the US citizens, who already voted in local elections, voted to extend voting rights to non citizens. The US citizens who were outvoted sued. But they lost the case because the policy did not "harm" them specifically, it affected all citizen voters equally.
If I was in DC I would vote no to this kind of thing, but since I'm not, and I don't do business there, it's none of my business.

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