This is from an article in the National Journal by Ronald Brownstein and Janie Boschma; note that the "education" referred to in the chart is the proportion of whites with four-year college degrees in each district. A district is high or low if it is above or below the national average on those measures:
And here are some details:
The core of the GOP majority is the party's crushing advantage in the final quadrant: "the lo-lo" seats where both the minority population and the share of college-educated whites trail the national average. In the new Congress, Republicans will hold 150 of the seats in this quadrant, compared with just 25 for Democrats, an advantage of fully 6-to-1. These include a broad swath of districts extending from suburbia into rural areas across the South (such as those represented by Renee Ellmers and Virginia Foxx in North Carolina, Lynn Westmoreland and Doug Collins in Georgia, and Mick Mulvaney and Tom Rice in South Carolina); much of the Republican strength in border states (such as the seats held by Harold Rogers and Ed Whitfield in Kentucky, Sam Graves and Blaine Luetkemeyer in Missouri, and Marsha Blackburn and Stephen Fincher in Tennessee); as well as places outside the urban core in Rust-Belt states such as Iowa (Steve King), Wisconsin (Paul Ryan), and Michigan (Fred Upton and Dan Benishek).
The demographic characteristics of an entire district don't map perfectly onto individuals, of course. For instance, there are plenty of whites without college degrees who vote Democratic, and plenty whites with college degrees who vote Republican. But this does demonstrate vividly that Democratic and Republican elected officials increasingly come from, and do well in, different kinds of places.
