I strictly want to use chats at work to better myself professionally

So ignore everything else.

I don't think it's necessary to agree or disagree with political positions to correctly and successfully oppose someone posting those positions in a technical chat. If your colleague was posting articles about how "Voting is Good" and "How to Encourage Everyone to Vote" and so on, you could get fed up of that even if you, too, are pro-voting. Ditto "Drunk Driving is Bad", "My Cat is Adorable" or "It's Sad When an Old Person Gets Hit By a Car."

So come at it from this point of view. You're just tired of seeing this topic, that's all. You came for tech chat or tech support, not this thing, even if it's great and marvelous. You don't even want to suppress the topic, you just want to be able to choose whether to read that or tech stuff. Right? [Note: this does not require you to pretend you agree with these articles. Just don't mention that you disagree or that they upset you. Take a position of ennui, of not-this-again-in-my-tech-chat, not of How Dare You!]

So, if the Slack has an off-topic, these messages could go there. Or if your tech thing is called ABCGroup because you're all in the ABC group, it could be ABCGroup-offtopic. I did precisely this on a server I use: there's a channel for Organizers, who discuss organizing stuff in the group - upcoming events, things to do, should we make X an organizer, who remembers how much money is in the UK bank account - and another organizers-offtopic group, open only to organizers, where people can rant, sob, ask for help, and even share delights and wins knowing it's a very small audience of their peers.

Logistically, I would think replying with this suggestion next time one is posted is Step 1. It may be the only step required, especially if other people reply "good idea." If the channel gets created, you are making progress. Once the person keeps posting stuff to the wrong channel, use the usual Slack techniques, like @mods or whatever, to get it taken care of. Or you can even mention to your manager that despite the existence of ABCGroup-offtopic, this person keeps posting offtopic stuff to ABCGroup. Since you're not asking that they stop, only that they do it in the right place your ask is far smaller and more reasonable, thus more likely to be enforced by mods of the Slack or managers of company, and less likely to be held against you. Since you haven't brought your political position into this, it's also likely to be safer if it turns out your position is a minority one.

If this person is senior to you it's a very difficult issue.

Take an extreme example: say the company owner was doing this.

The simple fact is it is their company and they can do what they want (assuming they are not crossing local laws).

Unfortunately your only choice would be put up with it, or get another job.

As you say, this is a senior person. But unfortunately the situation is similar.

The fact is you have only three choices:

1. Explain the situation to a more senior person

2. Put up with it. Learn to totally ignore it.

3. Get another job.

Point 1 is difficult. If it's not an actual legal issue, the fact is nobody likes a whiner. It is perfectly possible the more-senior person may think it's no problem, anyway.

Can you adopt a "water off a duck's back" attitude?

I'm like the oldest person on this list and I've "seen it all". Say some company is paying my company 100 grand a month to make the next Facebook for Dogs. Say the client has an infuriating habit .. nothing illegal or harassing, just infuriating, such as spouting ridiculous political opinions constantly. Can you guess what I do? Exactly ... I just ignore it all the way to the bank.

I could not care less and would not remember it one second later.

when you're just starting out in your career this sort of thing can be annoying. try your best to just "work to work, and live to live". Take the money, go home, and enjoy life! It can take many years to master the skill, but try to adopt a "water off a duck's back" attitude.

The fact is unfortunately you do only have the three options mentioned above, that's it :/