WEST CHESTER, Pa. — At age 48, Patty Mapa can't remember ever voting for a Democrat for president.

Then Republicans nominated Donald Trump.

The substitute kindergarten teacher, who was shopping for fresh produce with her husband and daughter at the West Chester Growers Market in this Philadelphia suburb on a recent day, worries the billionaire businessman is "negative, just very divisive, and erratic." She's less than thrilled about casting her ballot for Democrat Hillary Clinton — "There's that dark little cloud" when it comes to trustworthiness, she says — but on this Mapa is certain: "I am voting against Trump."

The biggest swing in the American electorate this year is happening among white, college-educated voters like Mapa. They are a big and growing group — an estimated 23% of the electorate four years ago and expected to be a bit more this year — and they have voted Republican in every presidential election since at least 1952. Four years ago, Mitt Romney won their support by a solid 14 percentage points, according to surveys of voters as they left polling places. But in the latest Pew Research Center poll, taken last month, Clinton led among whites who have a college degree by 14 points.

That may be the most dramatic partisan shift by a major demographic group from one presidential election to the next in modern American history.