Ares96:
I actually thought I was going to be the first one doing this idea, until I saw Otherworld's. However, this should hopefully be different enough to still be interesting.
The basic premise of this is that of Decades of Darkness - Jefferson's death in 1809 leads to New England seceding, supported by the British, at which point the US begins the *War of 1812. However, ITTL Rufus King isn't shot, leading to less Federalist sympathy within New York, and so the *US are able to keep the west of New York, as well as New York City itself. With two more free states in the Union, the slavery debate still grows into a major issue, and Wilkinson finds himself having to compromise over Missouri's entry into the Union, with it becoming a free state on the condition that no other free states are to be admitted south of its northern border. In addition, Illinois and Indiana never repeal their free-soil status.
Later on, Jackson still comes to power, and the War of 1833 begins as in DoD. However, there is no Pennsylvania Rebellion, and so the *US is more successful, and eventually they are able to regain almost all of the Northwest Territory. A second compromise is now reached, whereby all of the new territory in the Midwest becomes free soil on the condition that the recent gains in Texas (which only go up to the Rio Grande ITTL, as Jackson sees no need to press further claims to restore *American prestige) are recognised as slave territory. As such, to compensate for the three free states admitted in the Great Lakes region, the three states of Houston, Tejas and New Mexico are admitted from the Texas-Coahuila Territory. Since the war is over quicker, Jackson doesn't seek a third term, and Hugh Lawson White gets elected for the Democrats.
White gets reelected in 1840, but dies only months afterwards, succeeded by his Vice-President, James Polk. Polk, in turn, is defeated in 1844 by George Dallas, who is succeeded in 1848 by Lewis Cass. Cass wages war against Mexico, same as in DoD, but due to less disorganisation in Mexico (the less disastrous Texian revolution leads to the Emperor remaining in power), only the OTL Mexican Cession is gained post-war.
When Cass signs an act into law that makes all of the Mexican Cession slave territory, in violation of the *Missouri Compromise (since parts of Utah Territory protrude north of the compromise line), the North is in outrage, and Congress agrees on a compromise that makes absolutely no one happy: California north of the 35th parallel is admitted as a free state, with the rest becoming a slave state and the Utah and Washington (Arizona and New Mexico west of the Rio Grande) territories remaining slave. To help sell this to the South, a Fugitive Slave Act is signed into law, which the Northern abolitionists come to despise.
Cass wins reelection in 1852, but steps down in 1856. The election that follows is turbulent indeed, as Jefferson Davis of the Democrats faces off against Cass's Vice-President, Sam Houston, and William Seward of the Freedom Party. Davis wins the election despite not having received a single electoral vote from the free states, much to the chagrin of the North, who realise that their interests will never be seen to in the present Union.
Around the New Year of 1857, nine states secede from the union, forming the Federated States, and choosing Abraham Lincoln of Michigan as their president. Since secession cannot legally take place without the approval of Congress, the states decide to be proactive, attacking the armoury at Harpers Ferry to try to start a slave revolt and tie up the Union army; this partially succeeds, as the Union army garrison is beaten back, and the weapons of the armoury distributed among the slaves on nearby plantations. After this, three more states secede, and the American Civil War begins in earnest.