Even knowing that the tariffs are virtually certainly a dumb idea brought about by a commitment to even dumber ideas, I need to know enough about it to estimate just what is the likely downside of going the way we are going.

It is, as you comment, a dumb idea, but let's look at what the less-dumb variants of the same strategy might entail.

I imagine there are ways tariffs could be done in a relatively prudent way. Say, a low rate applied to all goods from all countries, without special exceptions for loud or high-donation special interest groups, so it really did resemble a national sales tax that merely exempted locally made goods. Announced well in advance, phased in, and well enough organized and explained that there wasn't a huge reason for other jurisdictions to apply countervailing duties. This would still make the US poorer and its economy weaker, but in a modest and gradual way, and if done gently enough it might raise a useful amount of money. It doesn't typically reduce a trade deficit partly because it would drive up the dollar, but it might help forestall even further erosion of some local producers and might delay any day of reckoning for the government debt.

Another alternative would be to adopt a strategy actually suited to the main stated goal of the tariffs (well, the goal that could in theory be achieved), being re-industrialization in the specific areas which are prudently required for defence and critical infrastructure. The US might arguably have to do something to maintain a viable steel industry and chip fabs, but probably does not have to make its own clothes and toys. One could probably come up with an industrial policy that accomplished that large but relatively narrower goal without undue chaos, with some mix of tariffs, subsidies, tax breaks, or subsidized lending. It looks like the UK is about to re-nationalize its last remaining blast furnace, for similar reasons.

When it comes to the reactions of non-US players, this is all complicated by the observation that the tariffs are not really the biggest problem causing the chaos these days, which is really the stance of the geopolitics and the management thereof. Trust in the US has been holed at the water line, from rule of law to defence commitments to stable regulatory frameworks to treaties to basic economics to safe returns on bonds, but in particular the de-alignment with what was previously known as "the west" (Nato, WTO, Five Eyes etc). I saw an article that included a analogy from the past. "Lech Wałęsa once remarked, you can’t turn a fish soup back into an aquarium." In a slightly perverse way, that part is accomplishing one of the goals: driving down the value of the US dollar to make US exports more attractive.

Jim