The secessionists in New England, for whatever reason, manage to break away from the Union in 1812(and, perhaps, even take some of the northern upstate New York with them), after an even more disastrous war, although some loyal Yankees still leave for other parts of the Union.
Meanwhile, the South industrializes earlier than it did IOTL, but slavery still begins to take off after ~1830, which worries many Northerners; this only intensifies after Indiana essentially virtually legalizes slavery in it's southern counties in the early 1840s.
However, though, the real turning point comes when the U.S. fails to win the Mexican War, or Oregon, under a far less lucky James Polk(though Texas is absorbed into the fold mainly as IOTL); many Southerners accuse their Northern counterparts of political sabotage, while some Northerners shoot back with accusations that the Mexican War was an engineered attempt to expand slavery to California(which breaks off and forms it's own state, after Santa Anna's successor promises not to interfere in their affairs, as long as John Fremont's California assures the right of passage for any Mexicans who may wish to leave the new country.).
At the same time as the end of the Mexican War, many tens of thousands of immigrants begin to arrive on the shores of the United States.
Abolitionism develops roughly on schedule, timewise, but much of the main activity is actually in the states of Michigan, Ohio, Illinois, Iowa, Missouri and Indiana, and later Wisconsin, as well as the Kansas and Minnesota Territories, much more stuffed with abolitionists(and more radical abolitionists at that!) than in our world. The TTL Free Soil Party is founded in the state of Indiana in 1849, as it's the one Northern state most affected by slavery's reach; they win no presidential elections, but their presence is still noted.
The slavers, however, soon face a conundrum: after peaks in the late 1850s, slave prices begin to decline afterwards, and several of the more liberal officials in Virginia and Kentucky again consider the idea of emancipation, which angers the *Fire-Eaters, and worries much of the rest of the Southern establishment. Further complicating things are the highly taxing tariffs signed into law under two Whig Presidents(Daniel Webster and James Buchanan, the latter a significantly different political person compared to OTL.)
Meanwhile, the Free Soil Party merges with the progressive wing of the Northern Whigs to form the Liberty Party in 1862, under William L. Garrison, partly as a response to the formation of the Republicans between the more populist Northern Democrats and the more conservative Northern Whigs, 5 years earlier.
The Republican candidate in 1860, Millard Fillmore, wins by a narrow margin in a three way race between the Free Soilers and the Democrats(the Whigs did not nominate an official candidate and would cease to exist in 1862). Under Fillmore, the U.S. and New England sign a friendship treaty in 1862, further angering the Southern conservatives; he also decides to seek better relations with the U.K.(as New England had done some 20 years earlier)
Fillmore does little about slavery himself but there is a silver lining: no Fugitive Slave Act exists ITTL. But the tide against slavery continues to grow, and the state of Indiana bans slaveowners from any residency in the state in 1862, also granting all African-Americans in the state full voting rights. Indeed, the deepest divisions actually begin to develop in Kentucky and Virginia, with many in the east, and west, respectively especially unfriendly to the planters in general.
And when Lovell Rousseau, the current liberal Republican governor of Kentucky(and a respected war hero), signs into law that effectively starts an emancipation program in the summer of 1867, it makes waves across the country; but it also makes the rest of the South fearful(despite the fact that no major violence breaks out in Kentucky in the next two years, apart from sporadic attacks by white racists).
William C. Rives, the governor of Virginia, follows a similar path just a year later(and only mainly to appease the western and far northern counties, on the condition that African-Americans be encouraged to leave the state, whether for other states, or outside the country), sparking much outrage amongst conservatives in his state, including even a massive riot in Norfolk.
But the final straw comes in 1868 when Salmon Chase, a liberal Illinois Senator with major anti-slavery leanings, manages to win the Presidency in a four-way race between his Liberty Party, the Republicans, the Democrats, and the Constitution Party headed up by John Bell(some of the Republicans and northern Democrats ultimately decided that the man who won the most popular votes was the one who should have the White House).
And so, with that, the South begins to drift away from the rest of the country, and when General Robert Rhett massacres dozens of protestors in April, 1870, in Richmond on the orders of the new governor(Rives was removed from office over charges of purported election fraud), Henry A. Wise, President Chase orders Federal troops to put down the insurrectionists. After a short battle Gen. Rhett is executed upon the retaking of Richmond, and known pro-slavery forces in both Kentucky and Virginia have their assets frozen-the Federal Government soon passes an enormous slavery tax(but only with many members of Congress absent); absolutely horrifying even many of the less reactionary Southerners and driving the Fire-Eaters over the edge. By the end of November, the Carolinas, Georgia, Alabama, and Mississippi have seceded(though Florida is heavily divided between the Unionists in the South and the rebels up north), and the states of Tennessee, Arkansas and Texas follow not long after; to make matters worse for the Union, a huge uprising of pro-Confederate forces has also occurred in Kentucky, Virginia, and southern Missouri, and Maryland and Delaware are having significant issues as well; the Civil War has begun.
The Unionists fight long and hard, but in March 1873, a crucial mistake at Gettysburg(cliche, I know) ensures the capture of Union battle plans-by September, Washington is almost encircled and the Union has even lost a huge amount of ground in Kentucky, and the Rebels are beginning to make major inroads in southern Missouri. By the end of the summer of 1874, it becomes clear that the Union cannot hold it's position, and U.S. General Grant surrenders to the Confederates on the Sunday before Thanksgiving, just outside of Baltimore.
The surrender is utterly horrifying to many Northerners and many in Pennsylvania, New York and New Jersey in particular fear that the victorious C.S.A. may feel quite inclined to brutalise them, with Confederate troops occupying all the way into Gloucester Co., N.J., and encircling Philadelphia. In a state of despair, Salmon Chase sees only one way out: in talks with N.E. President Lysander Spooner, he offers to allow the Yankees to occupy(protect) New York and northern New Jersey, with permission of the locals, as the Union Army has been largely destroyed(and much of the Navy sunk) and is largely incapable of defending the nation from any further attacks. Spooner hesitates, but out of concern for the safety of the people in the occupied areas, he agrees, but only if the governors and legislatures of these respective states agree on it(and a plebiscite held after 25 years to determine; the N.E. government also agrees to allow the Americans to keep their U.S. citizenship as well, if they wish. Congress allows it by a mere five votes, but not without major protests.
Ultimately, as highly controversial as it is, all three propositions manage to pass by a narrow margin(the northeast part of Ohio later joins N.E.) in the fall of 1878, and, by January 1, 1880, the U.S. is devoid of a coastline, entirely dependent on it's foreign allies for overseas trade, and with it's capital moved to the town of Peoria, Illinois.
There is, however, a silver lining-virtually nobody likes the Confederates, and the rump U.S. has plenty of friends amongst it's other neighbors; there do remain quite a few people in the former northeast of the country that would one day like to return to the Union once the Confederacy no longer poses an immediate threat to their safety, but as New England has no laws against states leaving their country(provided the secession is approved of democratically, of course), this is not likely to cause any significant problems(as in case of retrocession, the U.S. has already pledged to protect the citizenship of Yankees living in those certain areas). The C.S.A., on the other hand, is already becoming paranoid regarding it's neighbors and even with a thriving industry of it's own(for now), it still must face half a dozen enemies on either side of it, not the least of which is the now vengeful U.S., and if the Confederacy falls to sociopolitical unrest, or loses the next war, could the U.S. and their allies be able to take advantage of this? Could the U.S. one day regain it's much-missed Atlantic coastline? Only time will tell.....