This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. One who makes malicious charges and otherwise attempts to discredit an opponent, as in a political campaign.

n. One who casts aspersion, who insults. Especially a political candidate who makes negative statements about the opposition.

n. one who spreads real or alleged scandal about another (usually for political advantage)

"He calls himself a "mudslinger," and works with classical shapes that are functional and durable."

disparager    knocker    detractor    depreciator