Many seem to think of the ACW as an inevitability, citing the natural consternation between the North & South over the issue of slavery among other factors.
There are no other factors of any comparable importance. On every other national issue, the geographical sections of the country were divided. And none of those issues were regarded by anyone as an existential threat.
Many Southerners regarded abolitionism as the prelude to a slave uprising and the general massacre of whites in the South. (The spectre of Haiti loomed large.)
Tariffs and railroads and banking and so on were by comparison insignificant.
With a POD of 1850 (Ill leave it up to you to change the 1850 compromise or not) how long can the ACW be plausibly delayed and what might be the effects, not only for the war but for *reconstruction and the US in general?
The trick is both to keep the war from starting in 1860, and have it start later. The trigger was the election of an anti-slavery President, which initiated the panic. But it was also the rise to power of the Republicans, the first real anti-slavery party. Southerners were initially very worried by the Republicans, but over time that fear could be moderated with familiarity.
So Republicans have to be more militant than OTL. Here's a try.
Justice Taney dies in early 1857, before the broader
Dred Scott decision is issued. The Court does not overturn the Missouri Compromise restriction on slavery in the Territories.
The Douglas doctrine of "Popular Sovereignty" remains acceptable to the Deep South (as they have not been given anything better). Douglas wins the Democrat nomination in 1860. The VP nominee is Sen. Benjamin Fitzgerald of Alabama. A smaller than OTL faction of Deep South Democrats bolt, and nominate Robert Toombs of Georgia.
The Buchanan administration makes a last effort to force the pro-slavery Lecompton constitution on Kansas. As in earlier years, thousands of armed "Border Ruffians" from Missouri enter Kansas to rig the election. However, this effort meets resistance. 50 Ruffians and more than 100 Free-Soilers are killed, many in the Lawrence Massacre.
But it succeeds. Between suppression of Free-Soil votes and fraudulent ballots, there is a very narrow majority for approval of the Lecompton constitution, and this result is certified by the Territorial governor, appointed by President Buchanan.
William Seward of New York wins the Republican nomination. The convention, goaded by Kansas to fervent anti-slavery feeling, nominates abolitionist Cassius Clay of Kentucky for Vice President.
The remnants of the Whigs assemble as the Constitutional Union Party, but cannot agree on a candidate. Most Southerners finally agree to endorse Douglas, but a northern faction nominates Edward Everett of Massachusetts.
Douglas comes out swinging to both sides. He savagely denounces the Kansas fraud - which has shocked even many Southerners. He also attacks Seward's alleged "radicalism". Clay's unquestionable radicalism on slavery is also alarming.
In November -
Douglas carries TN, NC, VA, LA, KY, MD, DE, NJ, IN, IL, CA, OR, and OH, and PA (154 EV), and becomes President. This is in due to significant old-Whig voters in the North supporting Everett instead of Seward, old Whigs in the South backing Douglas, and the failure of Toombs to unite southern Democrats.
Congress fails to act on Kansas statehood before the term ends.
Rep. Owen Lovejoy, whose brother was a famous abolitionist martyr, is appointed to succeed him by the Republican governor of Illinois.
In the summer of 1861, Douglas dies suddenly of typhoid fever.
Fitzgerald as President adopts a strong pro-slavery line. The Supreme Court upholds the Lecompton vote. In the 1861-1862 session, which starts in December, Fitzgerald musters enough votes from Southerners (including ex-Whigs), and northern "Doughface" Democrats to get Kansas admitted as a slave state. (Additional Democrats were elected in 1860 on Douglas' coattails.)
Outrage over Kansas sweeps the North. In the 1862 elections, Republicans gain hugely in the House (assisted by the 1860 reapportionment) and also in the Senate. The lame-duck Congress passes a law which would prevent a state from altering its initial constitutional position on slavery without the consent of Congress (based on the idea that the state's first constitution is a contract with Congress, and aimed at preventing such a change in Kansas).
In the 1863-1864 Congress, the House repeatedly votes to repeal that law, but action is blocked by the Senate. The law is upheld 5-4 by the Supreme Court. Northern opinion is increasingly outraged, and open abolitionism more widely preached.
Southern Fire-Eaters warn of the increasing dominance of the increasingly hostile North.
In early 1864, the Supreme Court decides
Pickens v. Cameron, ruling that slaveowners have a "right of sojourn" in any state or territory with their slave property. This massively inflames Northerners, who are now convinced that slavery will be made national.
With the Democratic Party in near collapse outside the South, it is dominated by Southerners, who nominate Jefferson Davis of Mississippi. (They recognize that President Fitzgerald is tainted by the Kansas issue.) In an effort to win over Whigs, they choose Reverdy Johnson of Maryland for VP.
Davis, though not a secessionist, is too punctilious to disclaim his party's hard-line pro-slavery platform, which calls for enforcement of
Pickett, even though he realizes that he can't win on it and Lovejoy's election will trigger secession. No third-party candidates appear to split the vote.
In 1864, the Republicans nominate Lovejoy, and Charles Sumner of Massachusetts (himself a more recent martyr).
Despite the surge in abolitionist sentiment in the North, Lovejoy doesn't sweep the free states: there is enough alarm at his radicalism to throw California, New Jersey, and Oregon to Davis. He still collects 179 of 309 electoral votes.
11 slave states declare secession - all but Delaware, Missouri, Virginia, and Maryland. The last three elect conventions to debate secession. In all three states, there are non-slaveholding elements that are opposed to secession and block action (German immigrants in St, Louis, mountaineers in western Virginia and Maryland).
The war is on!