Kent County, home to the capital Dover, has been a swing county for most of the century. The state capital, Dover, is located here and only makes up about a fifth of the county’s population. Most of the rest of the county leans Republican. The only times Kent has gone Democratic were in Democratic landslides, 1980, 1996 (because voters were more receptive to moderate southerners Carter and Clinton) and when native son Joe Biden was on the ticket. Without him, Kent reverted back to its lean-Republican ways.
New Castle, the most populous of the three counties and part of the Philadelphia area, and also home to the highly influential DuPont family, voted the same way as the state in most presidential elections. The county was strongly Republican before swinging Democratic in the Depression, moderating after the war, and swinging to LBJ. However, New Castle reverted to a Republican PVI in the 1970s and 1980s, though it was now the least Republican of the three counties. In the 1990s, the DuPonts’ influence waned and the urban areas made a sharp left turn like many in the country, bringing New Castle firmly into the Democratic column and Delaware into a nearly reliably Democratic state.
Sussex has more in common culturally with the South, though it was not solidly Democratic like most of the South was in the 1930s and 1940s. While it had a Democratic PVI even in 1964 and did not vote for Goldwater, it swung sharply Republican in the Nixon and Reagan landslides. Sussex was also receptive to Carter and Clinton, but returned to the Republican column from 2000 onwards, not even voting Democratic with Biden on the ticket. Interestingly, Sussex had a less Republican PVI in 2016 than after the Nixon and Reagan landslides.
Here is the link for the Delaware PVIs.
Here are the maps.