I came to this website, just wanting to exercise my freedom of speech rights and tell everyone my opinion about [issue]. But people keep downvoting my questions and answers. Is this website infiltrated by the uninformed sheep who blindly believe [other opinion] and keep downvoting everyone they don't agree with? Do people use downvotes to suppress my opinion and further their own?
People don't downvote you because they disagree that your opinion should be posted on this website, they disagree that any opinion should be posted on this website.
Many people on this website will downvote any question or answer which is one-sided and opinionated even when they completely agree with the opinion. The reason is that such contributions do not fit the purpose of this website.
The "What topics can I ask about here?" section on the help center says:
Politics Stack Exchange is for objective questions about governments, policies and political processes.
It is not a place to advance opinions or debate, but rather for exchanging objective information about the policies, processes, and personalities that comprise the political arena.
To avoid getting downvoted for being too opinionated, try to stay calm even when writing about topics you have very strong feelings about. Any question should have the goal to learn more about how politics work. Any answer should explain the workings of politics from a neutral and objective standpoint.
When you are not here to learn or teach about governments and political processes and instead just want to spread the word about a political cause, then you are using the wrong website.
See also this related FAQs:
It seems highly unlikely that there there are not some people on Politics.SE who use downvotes, (and upvotes), to strategically promote and suppress opinions. Officially this is frowned upon, and can be a banning offense. Presumably some of the more artless serial downvoters have been banned.
The sarcastic characterization of "infiltrated by the uninformed sheep who blindly believe [other opinion]" is unfortunate, as it wrongly imputes serial downvoters to be necessarily oblivious, as opposed to proudly, fanatically, deviously, mercenarilly, or militantly, partisan. Put another way, it's no easy task to measure the online ratio of well-informed wolves to uninformed sheep. But surely this ratio has increased over the years, and funding for wolves has never looked better.