They're all Northeastern states imo.
They are below the Mason-Dixon line. They will always be Southern states.
Why does everyone think they're Northeastern, because they've been safe Dem since 1992? Because they're part of the I-95 corridor? None of those things matter, all that matters is whether slavery was legal there in 1860. Otherwise we'd be changing regional definitions every 5 years. The Mason-Dixon line is like the San Andreas Fault of the East.
Being on I-95, Acela, safe Dem since 1992, colleges generally being in Northeastern conferences, Baltimore and Washington not really following the development arc of Southern cities, Delaware pretty much just being a suburb of Philadelphia, density of a Northeastern state, they weren't part of the Confederacy, not really part of the Solid South, etc.