This isn't a US politics thing, it is happening everywhere. Politics has been extremely negative and hyper-focused on character assassinations since the rise of social media - generative AI is just the latest weapon being used in this new battleground. Even if there are party rules or laws introduced to prevent official campaigns and offices from doing this, it will still continue to spread via the public. We are now living in an age where anyone can do this stuff pretty quickly and easily and that will only accelerate as the technology improves.
EDIT: Some examples of generative AI being used in major elections this year: