Strong, confrontational:
* Diatribe: A bitter and abusive speech.
* Invective: Abusive language.
* Tirade: A long, angry speech.
* Denunciation: Public condemnation.
* Harangue: A lengthy and aggressive speech.
* Rant: A loud, angry, and uncontrolled speech.
* Rebuke: A sharp criticism.
More neutral, focused on argument:
* Controversy: A prolonged debate.
* Dispute: A disagreement.
* Debate: A formal discussion of opposing viewpoints.
* Argument: A reason or set of reasons given to support a claim.
* Contention: A statement put forward as a subject for discussion or debate.
Focused on writing:
* Treatise: A formal and lengthy written work dealing with a serious subject.
* Discourse: A written or spoken communication or debate.
* Essay: A short piece of writing on a particular subject.
* Pamphlet: A small book containing a short and often controversial argument.
The best synonym for "polemic" will depend on the specific context of your writing. Consider the tone, intensity, and subject matter of the polemic to choose the most appropriate synonym.