Biden stepped into further controversy along racial lines when he responded to a question about his viability in conservative Southern states by claiming, inaccurately, that his home state, Delaware, "was a slave state" in the early days of the republic.
Depends a little bit on what one means by "slave state" and by "early days."
There was slavery in many "Northern" states in the early days of the republic. One can read for example "The Crisis" by Winston Churchill (no, not the once PM). Of New Jersey, note: An exception was New Jersey, where the slave population actually increased during the [Revolutionary] war. Slavery lingered there until the Civil War, with the state reporting 236 slaves in 1850 and 18 as late as 1860.
UPDATE. See also "Parallel Communities: The Underground Railroad in South Jersey," by Dennis Rizzo.
Note Springtown NJ Cumberland County (near Greenwich)
Marshalltown and Salem
Dutchtown, Cootstown. "Jersey Dutch"
Saddlertown Camden County