What's the most undemocratic thing US politicians ever rammed down the public throat?

That’s a really good suggestion. It gets my vote.

Let’s distinguish between “undemocratic” and “unpopular”. If democratically elected politicians vote for a certain unpopular bill, how is that less democratic than voting for a popular bill?

If we’re talking about undemocratic, then the establishment of our current constitution certainly should count. A bunch of aristocrats got together to talk about problems with the Articles of Confederation, and we suddenly get this proposal to radically restructure the government.

It would seem that something “undemocratic” would mean something “unpopular” with, or " unwanted" by the majority. A majority would be simply more than 50%.

Conscription, most notably in the Viet Nam war, but also in the American Civil War. There were draft riots in the 1860s, and huge protests in the 1960s. Picking someone up by force, putting them in a uniform, and making them fight and die for you is never going to be popular, and in those eras it was especially despised.

Well, I don’t think most Americans were against it (or understood much about it) at the time of the Gulf of Tonkin Resolution. The percentage who agreed with the war was 65% in 1965, 28% in 1971 (when Gallup stopped asking the question). If the pols persist in an initially popular policy after the public has turned against it, does that satisfy the OP’s conditions?

Bear in mind, however, that any meaningful discussion of “public opinion” in the Reconstruction South must consider the opinions of blacks (who formed nearly 40% of the population) as well as whites. And the blacks were all for every bit of that, I’m sure.

Yes, but it was only a proposal. It had to be ratified by (by the standards of the time) democratic means. National discussions were deep and lengthy, in homes and in taverns and in coffeehouses and in the press, producing The Federalist and some long-forgotten anti-federalist literature, and no one can say those Americans qualified to vote did so with their eyes closed or were ignored by the pols.

This is a key point, and may lead to a novel way to see the question.

In 1861, Washington County, Mississippi, had a population 92% of whom were enslaved. If you can find a law relevant to slavery passed in this county prior to 1865, there’s a fairly decent chance that more than 92% of the population of the county opposed it.

Edit: A couple of notes. First, while the OP is discussing federal action, I don’t see that as an explicit requirement. Second, in modern times, I believe there are jurisdictions in which the population is overwhelmingly imprisoned, but the prisoners lack the franchise; a similar situation may have arisen in some such county.

If they ever heard of it, ignorant as slaves were so carefully kept. “What’s a ‘law,’ Massa?” Which raises the question of whether the “public opinion” of a sector of the public which has never had the chance to form an opinion on the subject should even be counted. OTOH, there’s no doubt that the slaves had a public opinion on slavery as such, and were generally against it.

When wagons were moved on public roads by internal combustion engines instead of horses, it was decided that driving about in such a wagon was a privilege, not a right. Hence the Drivers License and License Plates.

What makes you think it was undemocratic?

As for slaves being ignorant, I suspect laws created to enforce slavery conditions were communicated quite explicitly to slaves. Even laws that weren’t would probably become known among slaves: some were educated, some worked in the house, and communication among slaves was vital to survival.

I believe that was the Statesmen-Fellatory Conscription Act of 1893. It’s not much remembered now, as it only applied to Negroes.

I agree this might be a quite undemocratic law, but Prohibition drew strong support from prominent New England groups - just not necessarily Catholic ones. In fact, the movement largely originated among urban, New England ministers. At the time, of course, it was not uncommon to be a Progressive minister with decidedly secular ambitions. Many southern and westerners certainly did support the movement as well, of course. Nonetheless, the country was deeply divided over it.

As I understand it, the law was technically color-blind and only considered lip size. (:p:p:p:p:p)

The Sedition Act of 1918 (repealed in 1920) was enacted by Congress and had fairly wide public support, but reading it today (see below) is rather chilling:

72% supported the Iraq invasion.

Americans aren’t anti-war, they’re anti-losing.

A good example of how statistics can be misleading. Break the vote down by party and region:

A higher percentage of Southern Democrats than Southern Republicans voted for the measure in both chambers. A higher percentage of Northern Democrats than Northern Republicans voted for the measure. It is only because at the time the South was overwhelmingly Democratic does one get the misleading perception that Democrats were more likely to oppose the act. And what happened to all those old southern white racist Democrats? They became Republicans.

Not funny and not appropriate to this discussion.

[ /Moderating ]

The general mood of the American people was isolationist in 1940 but President Roosevelt built up American military forces and gave support to the United Kingdom.

As much as I admire Lincoln and consider him the greatest president ever, I have to say the suspension of habeas corpus and suspension of other constitutional rights during the Civil War have to top the chart.