Anti-slavery activists hoped that the 1832 legislation for gradual emancipation enacted by the Virginia state government would set an example and act as a domino to end up with the eventual peaceful abolition of slavery nationwide. And indeed it would be quickly followed by emancipation legislation in Maryland and Virginia.
But around the same time, the cotton gin would transform southern economies and cement slavery (which was once seen as a 'temporary, necessary evil', but was increasingly seen as a 'permanent, positive good') in place in the rest of the south. The Virginia legislation also caused considerable concern and consternation over the 'freedman problem' in other slave states - during the 1831-32 debate on slavery in Virginia, the question of what to do with freed slaves and whether to re-colonize them back to Africa was discussed, but eventually postponed for later discussion and never really returned to. And the Whig national leadership over the next 20 years preferred to avoid the subject and controversy of slavery altogether, instead focusing on trying to build the 'American System' of tariffs, 'internal improvement' infrastructure spending, and industrialization.
The matter of slavery would nonetheless seep back into politics, with the annexation of Texas, and with the rise of the 'conscience Whigs' in the free states. The 1850s would see American politics turn downright vitriolic in debate over slavery, and with various controversies such as 'Bleeding Kansas', the caning of abolitionist and then-Senator Charles Sumner by pro-slavery Representative Preston Brooks, and the controversial 5-4 Supreme Court ruling on the Dred Scott case. In 1856, the Whigs for the first time nominated a candidate who took a stance against slavery, calling for the gradual emancipation, with compensation and recolonization, of southern slaves over the course of 50 years, with additional compromise in regards to tariffs and infrastructure in the South. Even this was far too much for the South, which was already trending towards the 'Fire-Eaters'. The pro-slavery Southerners would be further incensed by the 1856 election results-Democrat James Buchanan, a Northerner but sympathetic to the South and slavery, managed to win, but only narrowly. The political polarization between the increasingly anti-slavery North and increasingly pro-slavery South would continue to increase. After the 1858 House elections, the Whigs not only won back the House of Representatives but narrowly held a majority even if one discounted the southern pro-slavery Whig representatives (which further angered the South).
The elections of 1860 were a breaking-point for the United States. In April, the Democratic Party would split in two during the Charleston convention, between an aggressively pro-slavery Southern wing (which would nominate a ticket of John C. Breckinridge and William Yancey) and a moderate Northern wing (running with a ticket of Stephen A. Douglas and Horation Seymour). A month later, the Whigs would nominate the ticket of John C. Fremont and Charles Sumner, both supporters of more radical immediate abolition without compensation along with land reform and civil rights, which provoked a split in the Whig party as well, with Southern Whigs opposing land reform and civil rights at the federal level, and holding that at most, abolition should be along the lines of a much more moderate and gradual plan with ample compensation to former slave-owners. The Southern Whigs would join with the remnants of the Know-Nothings, running with the ticket of William Alexander Graham and Sam Houston under the 'American Unionist' banner.
In the end, Fremont and the Northern Whigs would win a decisive victory, failing to win a majority of the popular vote but beating their closest opponent by around 18%, while winning every Free, Northern state, aided by the controversy over slavery as well as support for Whiggish policies of infrastructure spending and industrialization. They would also gain a majority in the Senate for the pro-abolition wing of the party, and hold their House majority. Douglas, while coming in second in the popular vote, only managed to win his home state of Missouri. The Southern Democrats were decisively defeated, but did manage to win 8 out of 12 slave states. And the election of Fremont and Sumner would push the South to its limit. In December, South Carolina would declare secession from the Union, more states would follow. In April, secessionist militia would fire on the Federal positions at Fort Sumter, and the Civil War would begin in earnest
The initial change here, the legislation abolishing slavery in Virginia, didn't come from nowhere. In 1831-32, the Virginia legislature in OTL considered proposals for gradual abolition of slavery. It probably would have been difficult for any to pass, but let's say they did manage to get lucky and pass something like this. And then Maryland and Delaware, both of which had just a small amount of slaves and would find themselves geographically cut off from the rest of the slave states (and thus ugly on this map if they went pro-slaver) quickly follow suit
Slavery still cements in popularity in the slave states, but due to it not being so controversial in Virginia, Maryland, and Delaware, the Whigs (and later, the radical Northern Whigs) are able to remain competitive in those states, and are able to have some more successes from the 1830s to 1850s, though Buchanan still ends up winning in 1856 for simplicity's sake. The increased Whig successes, and the three additional Free states, help the Whigs move themselves and the broader public discourse more in the direction of anti-slavery ideas, and in the 1850s, rather than shattering like OTL, they remain coherent (though still facing internal struggles) and just become dominated by the Northern 'immediate abolitionist' wing (roughly similar to the Radical Republicans of OTL) by 1860. The Democratic Split still happens due to the solidification of Slave State support for slavery, but more of the country is Free, and more strongly leaning in that direction than OTL, so the Northern Whigs basically play the same role as the Republicans of OTL but perform even stronger
The Civil War could be rather shorter due to Virginia being a Free state. DC also is never threatened, nor is there any sort of 1862-63 campaigns that threatened the North on the scale of, say, the Gettysburg campaign. On the other hand, Kentucky and Missouri combined had ~750,000 more people than Virginia, so maybe they secede and make the Civil War of somewhat comparable scale to OTL, though still without really threatening DC or anything, and with Maryland and Delaware being more firmly pro-union