The heavy tariff would have never passed if the departed southerners had joined forces with the northern Democrats. And I doubt that even Republican support would have been as strong without suppression of a rebellion to finance. If you read the southern secession declarations they were dominated by the slavery issue. In their own words, tariffs were a minor irritant at most.
Not true. While the tariff would have had a harder time passing in the Senate (but even there it had the numbers to pass), the House by a large margin supported it. Remember, the Confederacy only comprised 11 states, whereas the Union comprised 23 states and the territories. Thus, the Southern states were outnumbered in both the House and the Senate, and the Northern Democrats would not have made a difference because they wanted to get their hands on the tariff pie, as well, for their own interests and projects. The simple fact is that Northern interests had the votes to impose the tariff, and the South had to either go along with it (which would have seriously degraded its economy) or secede. They opted for secession, as they believed the Northern states, by concentrating their revenue generating efforts against the Southern states, violated the Constitution and thus that compact was by right dissolved.
Was slavery an issue? Yes, but not to the extent the revisionists would like us to believe. The South knew that the issue of whether or not slavery was to be legal was up to the individual states, which had the right to determine what it allowed within their own borders. They also knew that slavery in the United States itself was protected by the Constitution, and that absent a change to the Constitution slavery could not be abolished, and they were confident there would not be 26 states that would ratify an amendment abolishing slavery (of the 34 states in the United States at the time, 15 were slave states, and they would not have supported such an amendment). In short, the South knew slavery as an institution was not going to be abolished legally unless some of those 15 states agreed to it). What the South did fear, though, was possibility of insurrections by the slaves, stirred up by Northern firebrands (the Nat Turner Rebellion was seared into the memories of the Southern planters, especially those in Virginia where the rebellion originated, and in South Carolina where there were similar incidents, though not as notorious).
You know, some people today think the modern day equivalent of the slavery issue, as far as inflaming passions, is the abortion question. To an extent, I agree with that comparison. However, I think a more valid comparison would be to Second Amendment. The Constitution protected slavery, and the Constitution protects the right of the People to keep and bear arms. There were anti-slavery firebrands and zealots in the mid-19th century who vilified slavery and slave holders to an obscene degree, just as today there are groups like the Brady Bunch and the Soros idiots and their ilk who trash guns and gun owners to the nth degree. The South was willing to fight for its Constitutional rights; I believe there are a hell of a lot of gun owners who will do the same thing if and when the gun-grabbers both in and out of government try to take away THOSE rights.
Read my profile. I wish the Southern states had not seceded, but I understand fully why they did.