Van Buren seems to me like the original Reagan Democrat, meaning a patriotic Democrat whose party shifted so far to the left it left them behind, today's Trump Democrats another example.
In Van Buren's case he was a solid Democrat who slowly became more anti-slavery as Democrats generally became adamantly anti-abolition.
Van Buren did rejoin the Democrat fold in 1852, 1856 and 1860, but in 1861 near the end of his long life, he supported Lincoln against other Northern Democrats (like former President Pierce) who were "neutral" or supportive of Southern secession.
In essence, Van Buren died as a Lincoln Democrat, who valued his country and abolition more than Southern "states rights" to secede.
Martin van Buren owned a slave, whom he inherited from his father. The slave ran away when VB was a teenager. VB was always “personally anti-slave” as so many modern “pro-choice” people are, but his POLICIES were specifically designed to protect and preserve the slave system.
So what he did in 1848, like the woman in “Roe v. Wade,” its kind of nice after the fact, but way too late. He as much as anyone not only brought on the Civil War but saddled us with a two-party system based on spoils/patronage/money but also ensured (because of that) that government would inexorably grow.