Defamation. Is it a legitimate political tactic?

"Bush is Hitler!"
"McCain is Hitler!"
"Donald Trump is LITERALLY HITLER... but orange."
Yes. They will and literally HAVE smeared any Republican as a member of the now-defunct National SOCIALIST Party of 1940s Germany... as if we're trying to cram nationalized healthcare and welfare programs down your throats.
But where did this mechanism begin? Why not actively defame your political opponents? Should we?