I'm sorry, but I think you're going to have to let this one go.
There's been a several similar questions on the Workplace exchange, and while none are identical to yours, they all carry the same theme: politics creeping into the workplace.- How can I avoid divisive political issues at work?
- Is it reasonable for me to complain about an objectionable newspaper being ordered for the office?
- How do I deal with the political promotional items in the workplace?
Similarly, there's a theme to the answers: If it's harmful to the company, tell the manager. If that doesn't work, keep your head down. In particular, Masked Man gives an answer in the promotional items question:
Never let go of an opportunity to mind your own business. You have nothing to gain from getting involved in a confrontation over it. Ignore it and move on with your work.
I'm afraid to say I think that's the correct course of action here too. Throughout the course of your question, you've said
- all of the employees here are supporting the same political party
- everyone in the working hall except me welcome this news and start commenting on it
- All of the co-workers in the hall sit close to each other and keep discussing the political matters of the country
- that your manager said "Yes, I understand the situation and can't help it",
- people here are obsessed with political parties and affairs
It seems that your company has embraced this change in culture; that it's become baked in. At this point, then, what you're asking is "how can I reverse an accepted change in my company's culture", and I don't know that you can. You can either ask employee A directly to stop, but I think we both know that's not a likely prospect.
If you believe it's genuinely harmful to the company, then you should bring it up with your manager. Unfortunately, you have and he's effectively told you to deal with it. The next best resource would be HR, but I don't advise you turn there. There's a trope on the Workplace that HR is not your friend.
In one particular question on the topic, Dan correctly points out:
HR won't pick sides. They won't say if you are doing right, or if the other guy is doing right. Instead they will look if there are any legal issues and if they have to protect the company or not. Chances are though if you're constantly complaining, they'll just find a reason to fire you.
The alternative, then, is to poll your coworker's opinion on it. If there's support for cutting the political talk out of the office, then as a collective it may be worth bring attention to your manager again. This time, be sure to point to definitive negative effects of the conversations. If several people find it distracting, say "we all agree, it makes it hard for us to stay focused on our work." Don't present the issue as harmful to you; present it as harmful to the company.
However, from the context of your question, it seems most people in your company enjoy the conversations. That shifts your goal from changing only person A's behavior to changing your entire office's behavior, and I don't think you have a strong enough case to do so.
If I were you, I would stay un-involved, and get better headphones. If you'd like advice on how do best do this, I'd point you again to the first linked question; in particular, I think Monica's answer gives the best advice here.