Here's something a little different from the usual scenarios depicting events surrounding the Aamerican Civil War: an ATL where most things happen to occur in much the same way, but it's a world where A) secession is explicitly mentioned in the Consitution as "an inalienable right derived from Nature", and B) abolitionism gains much more popular support than in OTL in both the (Northern) USA and Britain. This ATL sees a Northern President faced with the deaths of multiple Supreme Court Justices during his term of administration, and he appoints men who later turn out to be more sympathetic to abolition than even many in the North had suspected. When a contentious case is ultimately brought before the court, the majority rules that slavery itself is unconsitutional.
Naturally, a call for Southern secession follows at once. The vagueness of the Constitution's wording, however, combined with the recent ruling of the Supreme Court, leads the Federal government to insist that if secessions go through, counter-secessions (e.g. West Virginia) must likewise be respected. Oh, and also... slavery has already been rendered void, so all blacks are now citizens, and they get to vote in the secession pleibiscites, too. This is furiously disputed in the South, but eagerly backed by Britain and France.
What follows is a rather brief war, during which the South is mostly occupied, and pleibiscites are held conforming to the federal government's terms. As it turns out, most blacks eagerly support the idea of pleibiscites on a more local level, since they aren't a majority in most (former) slave states. Thus, the result of the secession pleibiscites sees considerable areas of the South opt to secede, while several black-majority areas vote for counter-secession. The Trans-Appalachian regions vote to remain with the Union, since they are largely cut off from the secessionist Deep South by a belt of nascent "Freedmen Republics". (Also, the USA promises several Trans-Appalachian states some interesting border adjustments and a lot of federal investment in order to sweeten the pot.)
The whole affair is less than entirely clean, however. For starters, there are severe voting irregularities in several regions, most especially in South Carolina. For another thing, Britain (which is directly involved in occupying the Deep South), seeks to turn the secessionist regions into British clients, and ocasionally looks the other way during all this vote-rigging. Also, Britain supports the Texian insistence on gaining certain territorial areas. (And is backed in this by France, which in turn gets British support for its claim-by-proxy in the form of a Mexican demand for certain border adjustments in the West. This boon for Mexican pride, and the French aliance with Britain, allows the French to ultimaely succeed in propping up the Imperial regime in Mexico.)
By the end of the whole affair, slavery has been abolished; the USA is feeling somewhat shafted; the CSA is much-reduced but independent (albeit a captive market to Britain); Texas has chosen to become a sovereign republic again (considerably more self-sufficient than the CSA); the Second Mexican Empire has better odds for survival; the Freedmen Republics have become US clients/protectorates; most whites have predictably fled from said Freedmen Republics (either to the USA or to the CSA, depending on political loyalties); and most blacks have fled from the CSA and Texas (and quite often fro the remaining Southern USA as well) to the Freedmen Republics. By spring of 1864, the whole matter is settled.
The USA gains neither Alaska nor Hawaii, and becomes relatively non-interventionist. It's still an economic powerhouse, though: the loss of the seceded regions of largely offset by the fact that Northern political supremacy facilitates much more pro-industrial policies on the federal level. Texas is rather middling initially, but once oil is struck, there's a major economic boom. The CSA remains rather agrarian and relatively poor. The lower costs of living make this more bearable for the populace, although there's a lot of emigration. US companies have started to increasingly export their manufacturing centres to the CSA because of the much lower worker salaries. The Freedmen Republics are generally more successful, having diversified their economy (and having benefited from heavy US investment). Mexico enjoyed successive waves of liberalisation, and has become something of a Southern equivalent to Canada (as seen from the US perspective).
All in all, the avoidance of shit like Jim Crow, the example of successful republics governed by African-Americans, the need for the seceded Southerners to sort out their own affairs (without getting to exploit others), and the thriving Mexico have all led to a more liberal and considerably less racist North America. This includes more respect for Native American rights-- most obviously in the form of the semi-autonomous Sequoyah Free State.
The local plebiscites would probably be opposed constitutionally because there's also a clause about state borders being inviolable without a vote of the state itself, right?
Interesting scenario though.