Nope. Not the asshole.
If employer gets involved, point out that he’s the asshole for bringing up politics at work at a time of major polarization. You didn’t call him names for being a MAGA-tard; you just told him you weren’t going to sit quietly while you listened to him demean others. Eff that guy.
Your son has an excuse: he’s 18.
What’s this guy’s excuse?
Thanks asahi. I hope to God his political beliefs are just a phase and not set in stone. He just got his first real full-time job, so I’m hoping that will get him out of his echo chamber bubble and open his mind somewhat. I hope his co-workers aren’t all like the OP’s!
You were not an asshole, merely momentarily foolish for taking the bait. I might have done the same. There are very few reasonable alternatives in this situation.
Personally, I think we need more assholes like you. I aspire to be such an asshole. 
Maybe we could reframe OP’s question as, “Given 20-20 hindsight, how could this response have been improved upon? What would he, or I, have done different?”
Not for the purposes of beating yourself up, but so that all of us have a better template for when WE are in a similar situation. And I might do something different from OP based on my capabilities or lack thereof.
And that would depend on what you hoped to accomplish.
Pre-COVID, I went to a meetup where there was an attendee who in addition to not really fitting in socially with the group, was a monologist and a crashing bore to boot. Yes, we know you dropped out of college to marry your first husband, who was an alcoholic, and your grandson’s wedding was not as interesting to us as it was to you. And that’s only the beginning of it.
Just before Shutdown v. 1.0, I messaged the group’s de facto leader on Facebook and asked if we should, like, do something about her, like maybe tell her to shut up and let someone else talk. We both admitted to each other that maybe there are people who don’t want to be with us, and that we’re not perfect, but when people are not showing up because they know she will be there, something has to be done about it.
You did fine. 
I’ve noticed that with some zoom groups I’m in. One is a church Men’s Group, and one of the guys clearly doesn’t have anyone to talk to (he’s married, but his wife works, while he’s retired… I’m betting she doesn’t want to listen to his monotonous monologues when she’s exhausted from her hospital job).
I’m really at a loss. In “normal times” at group meetings, I’ve stepped in and said “Let’s give someone else a chance to talk.” But these days, a lot of people are just trying to cling to sanity, and I hate to take an outlet away from someone.
…
Speaking of sanity, I had this happen in a college class I was teaching. Had a schizophrenic student, also on the spectrum. He was a great kid, but quite a handful.
With his counselor, we worked out a system and I thought things were going well. Then I noticed three of the best students started missing class. When I reached out, they admitted it was because of That Guy.
So, what was done about That Guy? Tough situation, I know.
Looking back, I wish I had been more dipl omatic, or even just plain polite. I could have tried to change the subject, interrupted with something work related or even, “this test is taking too damned long.”. That would have prevented his being able to spin the story his way to other coworkers, which is what has happened.
If they know him at all, they may take his version with a big grain of salt.
You’re not an asshole, you’re my hero! 
You used your words and spoke your truth. Saying Hey I’m an officer in our local Democratic Party isn’t a lie, it’s not unkind; it’s a statement of fact. Saying I don’t want to listen to your name calling is what therapists call an “I-statement” which is how you are supposed to frame things.
A lot depends on how much clout you have in that group of coworkers and at your job in general, but no matter what rank, what you said was appropriate.
I wouldn’t worry excessively about that. Just be yourself and keep it up for a while. Your “best” self, I mean. Tread carefully but you have no reason to do anything different on the job. Eventually they’ll figure out the truth – or they won’t. Maintain your dignity regardless.
Social capital is a tricky thing. Don’t assume he has the upper hand because he “scored a point” of some sort. He hasn’t. He had the last word but that doesn’t mean he won.
I’d give you a B or a B+ on your handling of the situation. I think the improvement would come from intervening before he could keep going on and on, which in turn set you off. Nip it in the bud: “Whoa! Talking about politics or religion is always risky, but in this case, none of us can leave if we choose to avoid it. Let’s change the topic.” But I think any grown adult should know better than to talk about that topic in the way he did when he really doesn’t know his audience, especially given the political fireworks of recent memory.