As it's Election's time in UK and some other countries as well, we're used to listening politicians' oratory. During the speeches we often observe how politicians try to win public support by denigrating their opposition or laying blame on them at times even unfairly and unjustly. Is there a word for such tactic?; it may not necessarily relate to Politics alone.
You could say they are using ad hominem arguments, or engaging in ad hominem attacks. It generally means attacking the person rather than their arguments.
(edited after realizing Andy was looking for the word for denigrating, and not just overall lying)
They could be addressing Strawman arguments
This basically means they're deliberately presenting distorted, exaggerated versions of their opponents' arguments that are very easy to tear apart. For instance:
Senator Jones says that we should not fund the attack submarine program.
I disagree entirely. I can't understand why he wants to leave us defenseless like that.
Depending on the situation, many of these may apply :-)
The Synonym Finder - Page 662 Jerome Irving Rodale - 1978
misrepresentation ... torturing; canard, rumor, hoax, forgery; monstrous lie, the big lie, mendacity, SI. whopper, SI. barefaced lie, SI. dirty lie, SA shameless lie; defamation, scandal, traducement, calumny, calumniation.
Jeff's "ad hominem" phrase is sometimes known – to footballers uncomfortable with Latin, perhaps – as "if you can't get the ball, get the man who's got it".
"Demagoguery" (augurar was first) has to be the market leader here. One could also work in a reference to "bread and circuses" or "panem et circenses" It's in the accusative case because "give'em" is understood as preceding). Though this is not about trashing the oppo.
Shakespeare probably has some good terms for flattering the mob in "Julius Caesar" and "Coriolanus", I suggest you riffle the pages.
I agree that "polemic" is misplaced. It does carry an overtone of being "over the top", though, and the root ("war") reminds me of the "Philippic", or oration calling for national resistance to an outside threat.
Might I also suggest "calumniation" and "scapegoating"? Then you have "black propaganda" against your opponents. When this was done against the Jews in the Middle Ages, we call it "the Blood Libel".