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In the US, very infrequently does anyone ask anyone else who they're voting for. In fact, talking about politics can very quickly turn into a nasty argument.

You're also correct about generally being able to tell someone's political leaning.

Another thing is that the 2 parties (republican and democrat) are very clearly opposed and distinct. And every republican I know very strongly identifies with being a republican (and my guess is that they vote republican unquestioningly straight across the ballot). Surmise from that what you will.