I am not in complete disagreement, but the CC was the point at which slavery, essentially, became a running sore that infected (for lack of a better word) politics in the US for the next eight decades...and at every level, practically; there's a reason Kentucky (for example) changed from a frontier for free (white) yeoman to one dominated by slave-holding planters with aspirations of aristocracy. The wealth engendered by slavery was never going to go quietly, and as long as slavery had been sanctioned at the creation, as it was by the 3/5ths clause, those aspiring to such wealth would always be agitating for its extension.
The sectional conflict arose because of slavery; slavery was made a foundational issue (one could even say the cornerstone) of American politics because of the CC and how the delegates compromised over it.
The southerners were willing to throw down the entire "national" effort because of slavery; absent the 3/5ths compromise, the US would not have come into existance as it did historically.
Having said that, the 3/5ths and the compromise meant that the south would continually see itself as less part of the nation and more as something unique for the remainder of the century and well into the next; the entire concept of mastery became part and parcel of southern culture, which influenced every American election from those of Washington onward. There's a reason Virginia was the mother of presidents, and it was not because William & Mary was a better law school than Harvard...
Which was the reason why they lost out on all settlements on slavery on the Western territories? What political influence could they have over the North?
What political mastery? I'll repeat myself. The 3/5 compromise actually reduced their representation in the House and in the electoral college. They took it for lesser taxation, which really amounted to nothing since taxes weren't apportioned by population during that time, but by tariffs, which did not favor the south.
With or without it, they would be a minority and would be outvoted by the North. Just look at the votes for the outlawing slavery in Missouri in 1819. It passed in the House despite the disadvantage or advantage the 3/5 compromise gave the South.
And the fact that the government repeatedly voted for tariffs which benefited the North mainly and which the South considered unfair? Tarrifs which were mainly passed using the votes of the North?
How about slavery in the territories? Where was the South's political mastery? Read this post.
The Old Northwest was declared free. The old Southwest was part of Georgia, Virginia, and North Carolina during the time of the Constitution, and when they ceded it to the Government, there were already slaves there. So opening it to Slavery didn't actually expand the scope of Slavery, since it was already there.
The Louisiana Purchase was settled by the Missouri Compromise. By that agreement, only Missouri, Indian Territory, and Arkansas were allotted to the South. The rest were declared free. Again, it was to the advantage of the North.
Texas was admitted as one state instead of being divided into many slave states. Again, the gains of the South were limited.
Oregon Territory was conceded as wholly free.
The Mexican Cession was settled by the Compromise of 1850. California was admitted as a free state, permanently altering the Senate Balance in favor of the North. Texas was trimmed down in size. New Mexico, Utah, and the rest of the Cession might become slave states if the population voted for it. Which given the climate and the Mormon population, is unlikely. Slave Trade in DC was abolished.
What did the South get in return? A Fugitive Slave Law that was difficult to enforce.
There's a worthwhile point to be made that it was the very generation after the revolutionary one that "created" the sectional conflict, and the generation of the revolutionaries' grandsons that actively sought to destroy the nation their grandfathers had built.
I'm definitely a long duree type; in the human scale in the US in the Eighteenth and Nineteenth centuries, the great divide in politics was over slavery, and it came to the forefront with the compromises at the Consitutional Convention.
So, my take it even if the Kansas-Nebraska Act had not come into being, and something resembling 1850 had stumbled along, some issue - filibustering in the Caribbean, slavery in the (new) Southwest, federal expenditure on fugitive slave retrieval sanctioned by the Taney court, something - would have come along the South would have deemed a line that could not be denied or whatever, and conflict beyond the political breaks out.
Lincoln and Seward were not wrong; either the nation would extend slavery, or it would abolish slavery, or it would no longer exist.
Best,
My point is that Civil War along lines of slavery is not inevitable! There were filibusters in the Carribean during Franklin Pierce's term (remember William Walker?). It did not lead to a sectional crisis. Slavery in New Mexico and Utah was made possible in 1850, and it did not lead to the sectional Crisis. Heck, New Mexico being made into a slave state would be accepted by the North as part of the Compromise of 1850 in exchange for the Texas cession of lands, California being a free state, etc.
Fugitive Slave retrieval? It happened many times before 1854, including what I said if you read my previous post, Anthony Burns. It did not lead to a sectional crisis. And without Kansas Nebraska, the Taney Court will rule Dred Scott narrowly, dismissing the case for lack of standing.
If the Western Territories become settled, say New Mexico and Arizona becoming slave states, the Utah remaining a territory because of Mormonism, and everything north being free, then I can see nothing that would lead to a confrontation that would lead to Civil War, since the only thing that can cause a Civil War is secession, and secession would only come if the Republican Party comes to power, and it can only come to power if it exists, and it can only exists if the Kansas Nebraska Act becomes law.
I'll quote myself again.
"Let's say that Scott in 1852 does better. He won't win, but his performance elect a few more Northern Whig Congressmen. Which was enough to defeat Kansas Nebraska in the House of Representatives, since it only passed 113-100 in OTL, and all Northern Whigs voted against it.
So Kansas and Nebraska remained free territories, and settlers without slaves will settle it.
Let's say in 1854, the Whig Party, which did not implode, makes a comeback as is normal for opposition parties in midterm elections. Abraham Lincoln would be elected as a Whig in Illinois that year as Senator.
In 1856, the Whig Ticket of John Bell of Tennessee and Abraham Lincoln of Illinois would be elected over Franklin Pierce and Jefferson Davis.
They quickly organize the Western Territories, and admit Kansas and Nebraska as Free States, balanced by New Mexico and Arizona as slave states (even if the population is very low).
The free states still have an advantage of two because of California.
Then in 1858 and 1859, Minnesota and Oregon would be admitted free states.The South can't have an objection in them being free states since they are so far North. In return, the Transcontinental railroad would be built from New Orleans towards San Diego.
The big issue of the day would be the Mormon War of Brigham Young. And of course, Nativism.
Without the issue of Kansas and Nebraska, the Taney Court would decide Dred Scott narrowly, simply dismissing the case for lack of standing of Scott to sue. (The Court only ruled that way because of the climate following Kansas Nebraska, it felt it needed to make a ruling to settle the issue. Here, without any issue to settle, it might as well rule narrowly against Scott, without repealing the Missouri Compromise.)
With the territories settled without Kansas Nebraska, the Whig remains viable as a National Party. The Liberty Party and the Free Soil Parties remain small parties, unable to win National Elections. The Republican Party won't be born at all. Lincoln and Seward remains the partymates of Alexander Stephens, while Salmon Chase would remain a Democrat.
Without the Republican Party winning, the South won't secede. Without Secession, there would be no Civil War."
The scenario above is what would probably happen if you want to avoid a Civil War.