I don't fault someone for starting a topic and then not saying anything more. It was a good topic idea (though I have reservations or qualms about the word "stupid" in the title). Neosandy said her (his?) part, up front. Maybe she didn't really need to add anything further, to make her point. Or maybe she had done her part and the furtherance of her idea should be carried on by others.

To answer the original question ("Why do liberals think Trump supporters are {'stupid'/'very wrong'}"), and to make the answer understandable to both sides, we would probably have to examine fundamental concepts. "Exceptionalism" is one. "Power" is another. Power is even more fundamental and simple than exceptionalism and might be the main thing underlying it. The new question to answer is: Is there a principle more important than power? And then: If there is, shall we actually act on it? Something like these new questions would need to be answered, before we could disentangle the polarization that surrounds the Trump topic.

The same questions exist for religion.

(I'm not any certified scholar about any of this; I'm just saying what I think.)

There's personal exceptionalism and there's national exceptionalism, and other kinds too. But exceptionalism is a basic (whether good or bad) (I'd say bad) "principle" or "notion" common to all the kinds.

I find that the situation about exceptionalism (the national kind) and power is described well in the book "The True F**g" by Stephen Kinzer. The book describes the point in USA history in which USA expansion (or expansionism) goes beyond the North American continent, starting in 1898. The book describes a great "debate" about this kind of expansionism, lasting a few years, and then continuing in USA history to the present day. The book is very good; however, I still wish for a more satisfying explanation of why "power" is not the highest principle we should act on -- an explanation so clear that everyone would finally agree on it and act accordingly.

There are also practicalities to consider. Upon arriving at some principle, one has to then find a sufficiently practical way to implement it.

Where I am (middle-class American, not having to struggle to survive), it seems better to discuss principles first (since we are so lacking in that discussion), before discussing practicalities (in which we are not so very desperate anyway). Another approach would be to discuss practical situations first, and use them as examples, to illustrate principles, or from which to derive principles.