Looking at this CNN map—https://www.cnn.com/election/2018/results/house–it seems that except for thin areas of the northeast and west coasts and hispanic and black enclaves in the southwest and south, the country overwhelmingly voted Republican.
In my opinion, flyover America voted Republican because the "deplorables" want to defend Trump. They want to defend him for two reasons. One is that he spoke to their economic plight caused by the US corporations exporting their jobs, leaving the American workforce and middle class hard-strapped. The other is that the adoption of Identity Politics by the Democratic Party has made the Democrats the party that hates white people—especially white heterosexual males who are defined as the victimizer of minorities, homosexuals, and women. It takes a really stupid white person to vote for Democrats who see white people as the problem.