People have been impersonating politicians' accents pretty well for decades, mostly for comedy value, without tripping over sentence structures and pronunciation (unless that's the joke...)
Other than lowering the effort bar, I'm not sure what deepfakes add. Sure, it means someone can make Donald Trump reading copypasta[1] or Dubya rapping [2] without knowing how to do impressions or even speaking the language natively, but a competent voice actor could do a much better job of putting words into politicians' mouths, and one thing political ads don't lack is budget.
1. https://www.youtube.com/watch?v=LEzIAixNkFI
2. https://www.youtube.com/watch?v=LEzIAixNkFI