There is a lot of anger toward Trump in areas around Democratic cities, per WashPost: "If Republicans want to hold onto the House, they will have to compete in communities that had little to do with the working-class regions that sent Trump to the White House in 2016: affluent, white-collar suburbs of Democratic cities."
Why it matters: "Many of the most competitive House seats this year are in the tony bedroom communities of Los Angeles, Chicago, Denver, Houston, Philadelphia, New York and Washington."