I ask again, is there any evidence that such allegations have ever been taken seriously?
I can't see fake porn having any reach, except amongst low-IQ consumers of internet trash, who are not only susceptible to obvious propaganda, but are already politically aligned against the victim.
Other AI generated content may have more impact, but that's another conversation.
On the topic of people believing outlandish things on whimsy evidence: Plenty of people were convinced Hillary Clinton was running an underground child pornography ring in a pizza basement based on paintings hung in someone’s house.
Maybe they were all politically aligned against her from the start, or maybe they weren’t and it affected the 2016 election. How would we know?
While no poll is 100% accurate, as long as you make it statistically significant and introduce a good control (say multiple fake and real stories, with some of them positive too), you'll get good signal and ability to say exactly if it did affect the election.
You seem to have made your mind and no further discussion is possible.