The last forty or fifty years weren't typical of the last century or two. Recent years saw the growth of a secular progressive ideology on the East and West Coasts. These secular progressives were more hostile to traditional religious, patriotic, moral and familial values than earlier liberals or progressives. They weren't very many or very popular at first, even in those regions and in the Democratic Party. That explains McGovern's landslide loss. But as the secular progressives grew more numerous and more powerful, Evangelicals became more conservative and the South joined the Mountain and Plains states as a Republican stronghold. As that happened the Democrats consolidated their hold on the Northeast and the Pacific Coast. Some voters became true believers and others just didn't feel at home with the Republicans.
This isn't a situation that people would have recognized in 1880 or 1940. Slaveowners liked to say that Northerners were all heretics and unbelievers, and so did some Southerners in later years, but the average Northerner knew his neighbors and knew they weren't pagans or atheists, and if he didn't agree with all of them about religion, they had many other things in common. Midwesterners out on the Plains may not have had much in common with New Yorkers, but they had no more in common with Virginia or Mississippi planters. What you are missing is that the United States -- that is to say the Northern States -- had a common culture for a century after the Civil War and had more in common with each other than they did with the Southern states. That only began to change about fifty years ago when the mainstream culture began to collapse, and things are still in flux now.
Political coalitions within a country don't form the basis for a new country. It's said that the Democratic Party got started when Jefferson's Virginia planters hooked up with Aaron Burr's New York political machine. They both hated the Federalists, but there wasn't much basis for nation building between the two. And if you were a Great Plains farmer who resented being the tail of the Eastern dog would you really want to turn yourself into the tail of the Southern dog? Your whole theory seems to be that Southerners had reason to resent the big city New York merchants and capitalists. If New Orleans transformed itself into the great commercial capital, wouldn't far away farmers resent its power? Why would they want to submit to its power when they could form their own country? Once the one united nation broke up, there was no reason to trade one set of faraway rulers for another.
Those states would have flipped, and it would have left the Great Lakes/New York self interest zone which still controls our politics today.
You don't really live in today's America, do you? There's a metropolitan culture in New York City or Los Angeles or San Francisco or Chicago, but it's also in Atlanta, Houston, Charlotte and other big metropolitan areas. There are elites and there are non-elites, but I don't see guys in Staten Island or Upstate New York or Downstate Illinois or Ohio wielding great power. Probably bankers and moguls in Charlotte or Atlanta or Dallas have more say than they do in how things are done. Some Southern states now have at least as much clout in Washington as the Great Lakes states and their cities are more in tune with Washington's policies than Rust Belt towns.
As i've pointed out, there were only a dozen slaves in the entire New Mexico territory by the 1850s, and at that time no one was putting up any opposition to it.
And as I've pointed out, those were African-American slaves. Native Americans had been enslaved under the Mexicans, the Spanish, and the pre-Columbian Indian civilizations. Probably there were still some Indians who were essentially enslaved even under US rule. The region wasn't inhospitable to slavery and probably would have adapted to African-American slavery if the Confederates had taken over.
There would have been no great migration of slaves into the western territories. Those areas cannot support large scale slave farming, and slaves were too valuable in the cotton growing regions to waste on the western territories.
Where slavery was legal and slaveowners controlled the government, uses would be found for slaves. I believe the Confederate Constitution protected the right of slaveowners to take their slaves everywhere. That was reason enough for the Plains States not to want to join the Confederacy.
You missed my point though. You've said Northerners hated Blacks and didn't want to live among them and have to compete with them. If true, that's all the more reason why they'd want a national border between themselves and the South.
And why do these things always seem to develop in states with heavy port traffic and much contact with foreign nations and ideas?
Midwesterners out on the Plains may not have had much in common with New Yorkers, but they had no more in common with Virginia or Mississippi planters.
The vast majority of the populace out of the South was not Planters. They were just ordinary folk like those in the Midwest. Socially the groups of states I showed you would have all grown together over time and become a more or less similar voting bloc without efforts to keep them separated artificially.
had a common culture for a century after the Civil War and had more in common with each other than they did with the Southern states.
After the civil war, the Southern states were impoverished and many were desperately trying to get on their feet in the midst of crushing poverty.
I am referring to what would have happened in the absence of a war. The Southern population would not have been destitute and would have had similar incomes and concerns as the rural Northern populations, and they would have grown together in absence of war.
If New Orleans transformed itself into the great commercial capital, wouldn't far away farmers resent its power?
Oh absolutely. It is in the nature of man to resent and envy wealth. However, the wealth of New Orleans would have also been commensurate with an increase of wealth of these farmers because their dollars would go further without the protectionism instituted to protect the Northeastern manufacturers.
You don't really live in today's America, do you? There's a metropolitan culture in New York City or Los Angeles or San Francisco or Chicago, but it's also in Atlanta, Houston, Charlotte and other big metropolitan areas.
It is a sickness of big cities and concentrated wealth. Alexander Tytler pointed out that it is part of a much larger cycle of which most people are unaware.

And as I've pointed out, those were African-American slaves. Native Americans had been enslaved under the Mexicans, the Spanish, and the pre-Columbian Indian civilizations. Probably there were still some Indians who were essentially enslaved even under US rule. The region wasn't inhospitable to slavery and probably would have adapted to African-American slavery if the Confederates had taken over.
They had the opportunity to do so for half a century, and yet they did not. Therefore they would not.
Where slavery was legal and slaveowners controlled the government, uses would be found for slaves.
You are ignoring the economics. Unless a "use" could be found which would pay better than cotton farming, people would put their slaves into cotton farming. It's about money. People want as much of it as they can get with as little effort on their own part as possible.
I believe the Confederate Constitution protected the right of slaveowners to take their slaves everywhere.
As did the US Constitution, though it had been deliberately misinterpreted in such a way to give states the right to overrule the "privileges and immunities" clause in the case of slaves. I think the US Constitution was never intended to be interpreted in the manner it was when it was written, but we've seen the misinterpretation play over and over again in Liberal strongholds.
The Southern constitution simply made the point explicitly.
You've said Northerners hated Blacks and didn't want to live among them and have to compete with them. If true, that's all the more reason why they'd want a national border between themselves and the South.
A border is just a meaningless line. (as we are now seeing with our own southern border.) It won't keep people out, only proactive measures will do that.
The economics would have moved states to join the South, and those advantages would have overcame a lot of reluctance in other areas.
Again, it would have started with the borders states like Missouri, Kentucky, Tennessee, Maryland, etc.