Earlier quoted context omitted.
I think its a cultural thing. Most people in the US (In my very uninformed opinion, mostly based on television/movies), are very explicit in their political stand. In India, its assumed almost sinful to ask for political stance/ "Who are you gonna vote for" in the urban society. (It may be illegal too, not sure)
In the US, asking who someone who they're going to vote for is still taboo (at least in my opinion), they're just more likely to tell you their political stance. Plus, it's pretty easy to determine based on any number of things (do they talk about what they heard on the news last night? how do they view the current administration? government in general?). Granted, I'm not American, but I think I've been here long eno…
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.