I don't know where kt got that saying about the kicked dog. Maybe it's a regionalism from a different part of the US than where I live. I think that in real life a kicked dog is more likely to bite back, as Trump did.
In the famous Sherlock Holmes story, the key clue was that a dog did not bark because the dog knew the murderer. IOW, dogs tend to bark around strangers, a territorial defense response.
IOW, kt is claiming Trump defending himself is evidence that the Dems' attack - interestingly, kt admitted that the Dems' video was an attack, something the Dems have implausibly denied - was effective.