It's a repudiation of Trump and his lack of any finesse + "let's try something new".
People still think that the problems facing the US can be solved by elections. Trump could have coasted into the midterms delivering on a lot of what he promised with a reasonable economy, but either through hubris or other means, he started the war and completely screwed the Republicans.
Dead-eye Pete with 20 pounds of swingin' meat.