Fascists employ this technique all the time precisely because they know it is hard to refute.
“Fascists employ this technique all the time precisely because they know it is hard to refute”
Press secretary, Take note!
Or simply... “Consider the source”
Or perhaps source out a factual smear campaign against the offending originator of said attack.
A variety of tried and true savvy responses will quell the offensive.
Just the same as was successful against Roy Moore in Alabama; is being deployed against Paul Nehlen in Wisconsin; and is often used by (some) Freepers who attack anyone supporting liberty as being Pro-Drugs, Pro-Gay and Anti-Cop.
A negative cannot be proven.