Usually when you go the "I'm a fervent Stalinist" route most people will stay away, if that's what you want. Otherwise, what I tend to do is explain why that particular perspective is wrong, without mentioning specific ideologies. "Liberal democracy is flawed because you are choosing between a shit in your mouth or a piss in your ear, they are two and the same; the same shit, different smell. It's the illusion of choice, but in reality both groups represent the people behind. What we need is one party where specific members are removed, and where that party represents the people." For example, I didn't say, hell yeah, I love North Korea, but I explained the flaws.
I live in 
so this might not apply in countries that have more than two parties:
Most times I'm asked "Are you a Democrat or Republican?" and I answer "Neither, they both suck."
Most times I'm asked "Are you a Democrat or Republican?" and I answer "Neither, they both suck."
Around where I am, saying that is taken as membership with the American Libertarian Party. 
Missed chance to just say “No” without elaborating
It depends on who I'm talking with.
Most of the time I will ask questions digging into why someone believes the status quo narrative, or I'll sprinkle in facts that are counter to it. I try not to get upset, and I try not to directly out myself as a communist if I'm not sure the other person also leans in that direction. If anything I make it clear that I'm anti-colonial and anti-imperial, which to a keen observer would indicate I'm in the ML sphere but to those less politically aware or more liberal it's quite meaningless.
Saying no and walking away helps a ton of its just some random stranger. It's crude, but works.
Not sure what that helps or solves, though. Shouldn't we be promoting political discussion as much as possible?
If we're too scared to even show we're MLs, how do we expect to carry put a revolution
Not OP or the person you're responding to, but my response is that not everybody wants to be a part of political discussion. I mean, yes, of course we are ML and we would, in theory, like it if people were open to learning about that. But I think for some people it's just really tough. Personally I commend everyone who can do this kind of stuff in public because I know that I have the social anxiety of a hermit crab.
On a more general note, I think everyone picks their battles. Being that even mentioning communism can get you in a lot of social trouble. Even someone who regularly talks politics might just have a day where they don't want to deal with it. Those are my thoughts.
I just don't think a cosplay convention is the best place to have that conversation with a canvasser. OP has no interest in voting, and I don't think it would bring much merit to try and convince them otherwise within a 5 minute convo.
If they're willing to talk and ask good faith questions then I guess, but if I'm at a convention for leisure, I'd just wanna chill lol.
I do organizing IRL, and just prefer to seperste my lives, personally.
Win.
Only right answer. Do it in a polite way if possible, but if they're a clear piece of shit, go ham.
Win?
Dunk on them. Deploy rhetorical nukes. Use the power of material analysis to make their liberal idealism look silly. Treat it as a contest and come out on top. Win.
This is what I've taken to lately... If they're a religious conservative.
If they come from a less unhinged, though still clearly misguided, point or view, I play nicer.
I grew up in a conservative family, though, and went to a religious school. Which why I'm a Marxist atheist now. Which is also why I have no tolerance for religious conservatives and dunk on them relentlessly with their own Bible.
EDIT: I should add that I'm nicer to neighborhood liberals because I think they more often come from a standpoint of wanting what seems to be better for everyone than what we have now. Conservatives just want subjugation, so fuck them.
Win.
“Own the libs with facts and logic” but with real facts and logic instead of rhetorical flourishes and strawmanning.
Just say the following:
"The maoist uprising against the landlords was the largest and most comprehensive proletarian revolution in history, and led to almost totally-equal redistribution of land among the peasantry."
Like another comment here touched on, if I'm not totally sure of the person's politics I'll usually just criticize the US/west from an anti-imperialist angle. It's useful because actual comrades will quickly pick up on what type of person I am (and maybe agree or make similar comments themselves) while other people will usually either politely nod or change the subject.
If it's a situation where I really don't want the people within earshot to know about my politics then I keep to uncontroversial statements. Like when the Ukraine conflict kicked off last year my coworkers were talking about it and while I wanted to say something about liquidating Nazis, I just said I felt bad for the innocents caught up in it and I hope it ends soon. Most people aren't gonna give you shit for saying "I don't want innocent people to die" lol.
I just focus on contributing questions to the discussion instead of opinions. Once someone gets started talking on politics they're usually forthcoming.
That puts you in a more informed position to decide about revealing your own stance or not.
I usually take all sides so nobody can tell my beliefs. Just go full cynic and troll them.
I certainly don’t do that but it’s fun when you know enough to make counter points against your own argument and then debunk them. Then you really seem in control of the debate.
proceeds to anticipate all counter arguments so perfectly that you’re just reading a book of theory for an hour
If you want to avoid it, just nod along or say you don't follow politics
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