I had a friend run for state legislature in the 1990s, his opponent sent people around targeted neighborhoods passing out fake flyers for my friend's campaign. They used his name and photo, but attributed positions he did not hold to him.
I imagine before cheap printing, it was gossip. I'm sure it's been the case since democracy started.
In pre-Internet days, it was difficult to get someone's views in their own words, especially in real-time. All information was filtered through various sources before being passed onto the consumer. Now, it's straightforward to check their website, or their twitter, or their youtube.
This, of course, is not perfect and requires a root-of-trust involving Google and DNS, but I'd certainly take that tradeoff versus Guy On The Street asserting things.
Solution: run ad blockers and noscript while they're still legal. Install them on all family members devices.
It's entirely possible to fake a real-time video interview for instance using voice synthesis and mapping the face of your target upon your face. It's frighteningly convincing.
It's pretty good, but not difficult to tell that it's not real. The bridge between pretty good and perfect when doing this type of thing is a very, very difficult problem.
It also requires substantial amounts of high quality, up close footage of the person. This may be work for major politicans, but is much more difficult for the vast majority of people.
I don't disagree that it will become more of a problem in the future, and in many cases "pretty good" will be sufficient, but I think people are overstating how much of a problem it will be.
Video material can be produced at will if a wealthy nation or corporatiom wants it and the model will be significantly better if they train it on specialized hardware.
I don't think that the average person has to worry about his face being stolen. I think that it's possible to produce extremely good footage if somebody is sufficiently motivated to fund a smear campaign. It won't hold up in court, but it doesn't have to either.