Not the usual sort of question I answer, but I'm from New York's capital region and so I care about Martin Van Buren! Calling him an abolitionist is a bit much, as his career shows a considerably weaker stance on slavery than that. ("Abolition" meant not just being personally anti-slavery but actively wanting/trying to end slavery.)

Following the turmoil of the Missouri Compromise of 1820 (in which Missouri was admitted to the union as a slave state in exchange for some federal restrictions on slavery elsewhere), there was significant feeling that the country was dividing on regional lines - industrializing north against slaveholding south. Van Buren was among those northern senators who agreed to the compromise, and he was quick afterward to try to strengthen ties between the wealthy, slave-owning southern elite and northerners who were willing to overlook the "sectional" issue of slavery. The efforts of his faction to keep the south happy led eventually to the election of Andrew Jackson in 1828 (as the first Democratic candidate), an obviously racist and proslavery presidency. As Jackson's Secretary of State, Van Buren worked to prevent emancipation in Cuba, and as his vice president he'd been in favor of a bill that would allow abolitionist literature to be seized in the mail. As president, he opposed the abolition of slavery in the District of Columbia and was in favor of the kidnapped mutineers of La Amistad, an illegal slave ship, being re-enslaved. These are not the actions of an abolitionist. (He also continued Jackson's harsh anti-Native American policies. He was not a good man.)

The early Democratic Party was riven with conflict over slavery, with the heads of the party emphasizing the need for unity between north and south, and the antislavery Democrats arguing that true Jacksonians would follow the will of the people. New York was a particularly difficult state when it came to the question of slavery: much of the population was your standard individualist northern small farmers, but there was also a more conservative class of pseudo-aristocratic landowners much like the plantation owners of the south, who had owned enslaved people until fairly recently and were a significant political force. The basic issue that divided the Barnburners (radicals) from the Hunkers (conservatives) was free soil: whether new slave states should be allowed to enter the union. The north in general was becoming more and more anti-slavery, which made it easier for Van Buren to ease off of his "unity with the south" ideals to keep up his popularity in his home region.

While Van Buren and his cronies like Silas Wright (whose premature death in 1848 would set off wild mourning) had initially occupied a fairly centrist position, accused by northerners of being too fond of southern institutions and by southerners of wanting abolition, following the Mexican-American War in the mid-1840s, the extremism of the south - which was insisting on new slave states specifically to balance political power - made their firm stance against allowing new states to have legal slavery very radical. Van Buren and the Barnburners would spin off into the (unsuccessful) "Free Soil Party" and eventually join up with the Whigs to become the Republican Party.

For further reading, I would suggest Martin van Buren and the American Political System by Donald B. Cole (1984), Martin Van Buren and the Emergence of American Popular Politics by Joel H. Silbey (2005), and Jacksonian Antislavery and the Politics of Free Soil, 1824-1854 by Jonathan Halperin Earle (2004).