Northern victory is probably a given barring foreign intervention, which is both unlikely and possibly irrelevant given the inherent difficulties in invading the continental United States, the north is more industrialized and has an even larger population advantage in the 1880s-1890s than 1861.
With the Third Republic in power in Paris for 20 years by 1890? And the Great Reform Act of 1867 being the Law of the Land in Britain for a generation? North America isn't seen by the British and French as Zululand or French Guyana.
While one could expect the "Arizona Territory" to probably go with the South (along with Oklahoma/the Indian Territory), pretty much every other territory and new state will wind up with the Union. And by the late 1880s to early 1890s, that's virtually the entire Lower 48 outside the Confederacy and the American SW.
That said, this time period is where technology begins favoring the defender, so the South has an advantage here, but probably not a decisive one.
It might be more brutal than OTL's ACW given more machine guns and possibly trench warfare, could lead to an earlier understanding of how 19th century tactics and 20th century weapons don't mix.
If the South couldn't make reliable Henry, Spencer, or even Sharps rifles in the ACW, I don't think a separation of 20 years will give them the industrial revolution needed for machine guns, which were the absolute state-of-the-art weaponry at that time. Gatling guns, maybe. Poor Gatling guns.
The problem is that the demands of the North and the South were fundamentally incompatible, and that the opening of the West to settlement exacerbated the whole thing. Sure, you could try a few more Missouri Compromise deals, but they were bandaids on a festering sore. I don't see how you can postpone the Civil War much without doing away with it completely.
Good point. In fact, since the end of the John Adams Administration, the South managed to get either a Southern President, a Southern Vice-President, or Southern-sympathetic Northerner President clean through to the end of the James Buchanan Administration. That's 13 for 13. Quite a run.
Unfortunately it also led to the idea that somehow the Executive Branch "belonged" to the South. Like the Supreme Court and the US Senate.
Sound familiar?
I'm just going to note that this it what I'm working on in my
TL and I would say that on balance, the north has even more of a preponderance in industry and population, but if the south figures out how to use the new power of the defensive (in other words, a war-long Overland Campaign

) then it could be a
very unpleasant war.
Perhaps. There is the imponderables of American defense spending, especially naval, with the country not suffering a crushing ACW debt from the 1860s, and how much will America continue with its preoccupation with its own development? With all these resources freed up, you could see a quicker settlement of the Old West, such that the Plains Indian Wars end much more bloodily for our Native Americans.
The problem for the South in such a war is logistics. The North has the industrial capacity to make and deliver large quantities of both weapons and ammo the South however.... Machine guns go through a lot of ammo fast and I doubt the South has the capacity to make enough ammo to keep a lot of machineguns loaded but the North would.
Agreed. Guns are one thing, but oh those bullets. IIRC, Shiloh for the South was crippling in that they lacked enough powder and bullets for finishing the battle on the second day.
The more time goes on the more slavery starts to disappear Southwards. By the time the Civil War started slavery was pretty much moribund in Delaware and Maryland and was fast vanishing from Kentucky. Missouri was a giant mess.
Delaware would have abolished it long ago were it not for the Missouri Compromise.
Central Maryland was still fire-eating pro-slavery. Otherwise you wouldn't have Mosby's Confederacy.
Kentucky didn't pass the 13th Amendment until 1976! That 19- not 18!

Kentucky was a more Unionist state, but it was also a "One Drop Rule" state. Not exactly Abolitionist.
In 1850 Missouri was a solid Slave State. Then after a decade of flooding German refugees settling in, 2/3rds of the state was solid Abolitionist and 1/3rd Pro-Slavery. Unlike most other states, Missouri was more of a solid dividing line (like Virginia), with the southern 1/3rd being the Pro-Slavery portion.
By the 1880’s I’m betting that more modern farming techniques have appeared and slavery is vanishing from Tennessee and Virginia
In Knoxville TN, the future WV, and SW VA, yes. The rest of these states were too tied up in the peculiar institution. It wasn't all about field hands. There's House Slaves, Tradesmen, and...women.

THAT last issue will keep slavery going forever, no matter HOW many tractors get bought.
perhaps even from parts of North Carolina
In their defense, during Secession the NC governor openly admitted it was not by choice but by recognition of the facts of geography. Which alone amoung the Southern states he tried to surrender his state to Sherman when Uncle Billy reached the state line. Sherman refused, but his boys DID manage to behave themselves

in NC compared to what they did to SC and GA.
I doubt that very much. Northern Arkansas in the 1880s-1890s could probably convince itself to join secession just on antipathy alone.
As a result if the Civil War gets delayed to the 1880’s – 90’s then the Confederacy starts off a lot smaller and a lot more doomed to rapid defeat.
I think it also matters just how much of a CF the Confederacy makes of Secession. Like, do they kill any number of Union troops "trapped behind enemy lines", or something even crazier than that? IDK.
Pretty much agreed here. The later they seceede, the sooner they'll get mudstomped. If they seceede in the 1850's, on the other hand, it'll be more likely for them to win.
Particularly as the opposition parties are in such a mess. HOW many parties were created and destroyed from 1840-1856?