The Cook report said the vastly unpopular healthcare bill--roughly 17%--was a suicidal risk for the GOP and that this midterm kooks like another wave election. Solid GOP races shifted to lean and leaning shifted to toss-up. Add the 20 seats that turned to Dem and you are looking at the change in the House.
Of course, the Washington Post explains how it isn't quite true about the House.
The New issue of the New Yorker has a long piece on how to get rid of Trump. Buried in the piece is an analysis by a conservative pundit who points out that Trump's current approval rating indicates a 48 seat shift in the House. Here's hoping.