Mamdani will be sued. It's a 1st amendment issue.
I am not a lawyer, but this seems unlikely. Federal law prohibits "false advertisement" which is understood to include misleading advertisements. Regulators can and do restrict certain types of commercial speech, and this kind of restriction has survived First Amendment challenge.
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False advertising does not have first amendment protection, surely. And requiring potentially misleading AI images to be labelled surely doesn’t infringe.