Tell HN: Disagree ≠ Downvote
Downvoting should be used when someone says something which is unconstructive, offensensive or antagonizing (bait).
Opinions which differ from yours, especially about subjective topics such as politics, should be encouraged. It's true that opinions can be unconstructive, offensive and/or antagonizing, so your you do have to use your judgement.
What to do if you don't agree with a comment? Either skip it and move to the next comment, or reply with your own thoughts on the subject. Even if the person is factually wrong, replying with a correction (or upvoting an existing correction) is far more constructive.
As a general rule, if you aren't sure, simply move on.