Why is there such a sharp political divide between Chester and Lancaster Counties PA?

[President Punxsutawney Phil]

There's still some conservative rural areas in the southern half of Chester County. The most densely populated parts of the county have tended to be along a corridor in the central part.

(Wikimedia Commons)

[President Punxsutawney Phil]

% with a Bachelor's Degree 25 or over

Lancaster County 27.3%
Chester County 53.0%

What was the percentage of bachers or over in Chester County ten years ago?

[President Punxsutawney Phil]

Chester County

Irish  23.5%
German  23.2%
English/American  15.8%
Italian  15%

Lancaster County

German  37.2%
English/American  14.5%
Irish  10.3%
Italian  5.5%

And if anything these figures understate the difference.  In Chester County most of those with German ancestry I suspect are essentially WASPs, and not culturally Pennsylvania Dutch.

Good point.