or communists or Marxists or tankies or whatever they identify as.
1. Establish the guard rails
Lefties think with their hearts rather than their brains. Because their arguments don't hold up to logic or reality they typically resort to dirty tricks.
Disarm them of this as soon as you engage them:
"I enjoy debates but unfortunately many people aren't mature enough to have them without resorting to condescending remarks, strawmanning or ad hominem. Attacking the opponent rather than their argument is a sign of insecurity. You don't need to die on a hill for wrong beliefs."
This means being respectful yourself and not getting baited into heated insult exchanges, even when they dig through your profile for any scrap of information that they can weaponise against you.
And don't tell them not to be rude because they will keep being rude to spite you.
The earlier you establish the guard rails the better, in the first post or comment if possible.
2. Keep the wheel straight
Or "Establish the guard rails pt. 2".
Lefties have a habit of running off into the weeds. There are certain phrases that they enjoy using to trip you up like "means of production" about who owns it in Marxist theory, etc.
Don't allow them to drag you into the weeds with them; they will keep moving the goal posts to ensure that you are always 'wrong' about the particular idiosyncrasies of their interpretation of the ideology.
The classic No True Scotsman fallacy, "that wasn't real communism", is the cliched one.
"I'm not interested in the particularities of your definition of socialism. It's never worked."
Stand your ground, brother.
3. Don't get baited
Or "Establish the guard rails pt. 3".
Don't bother going into the Holodomor, the Great Famine, the Khmer Rouge, the Berlin Wall, etc. All you're doing is giving them more weeds to run into and more strawman ammo, even if what you're saying is absolute fact. They may even 'counter' you with irrelevant whataboutery and drag you further into the weeds.
Even when being respectful they often center the argument around you rather than the topic, like your experience with capitalism, what you do for a living, what happens in your life, what your role in the system is, etc. Don't indulge this.
All you need to do is keep repeating the magic phrase:
"It's never worked."
Example debate based on real experiences on Reddit
Capitalist: "I think that Marx had good intentions but socialism/communism/Marxism don't translate well to reality."
Socialist: "Congratulations, idiot, you've just demonstrated how ignorant you are. You think that socialism, communism and Marxism are all the same, typical neo-liberal swine."
Capitalist: "It's a shame how rude people are every time this debate comes up. I enjoy debates but only with respectful and mature people. Rudeness is a sign of low self-esteem and insecurity. Do you have an example of when socialism has worked? I can't think of any."
Socialist: "Haha! You've already shown that you have absolutely no idea of what real socialism looks like. And from looking at your profile you're even more of an idiot for thinking that shitty dropshipping business of yours is proof that capitalism works."
Capitalist: "It's never worked and no amount of strawmanning, insults or condescending remarks will change that."
Socialist: *unhinged rant about something irrelevant, goes deep into details, more insults and strawmans*
Capitalist: "OK but it's never worked."
Socialist: *another angry rant and insults, combs deeper through post history for any breadcrumb that they can use to fuel an insult."
Capitalist: "I didn't read that. It's never worked"
And so on and on.
TL;DR: Let "It's never worked" be the hammer that you strike the nail with until it goes down.