Why the GOP didn't do so well this mid term.

Trump is like a cancer on the GOP and they did it to themselves. They did it by teasing the rabid southern white bigots with dogs whistles for over 50 years known as the Nixon Goldwater southern strategy. The southern strategy that was used to transform the GOP from the party of Lincoln to the party of Strom Thurmond an old style southern democrat from the earlier 20th century who supported segregation.

In recent year, Trump and the Cancer he is to the republican party stepped teasing the southern bigots and started rallying them at his rallies.

Unlike the mid 20th century, the younger generation not only understands all the dog whistles, even when not directed at them. They tend to be equally as offended as the folks Republicans seek to marginalize as most younger voters grew up exposed to far more diversity and better education then their parents and grandparents were. It's why Republicans are trying to dismantle the education system in which millennial generation have been brought up in and continue to be brought up through. It's why some Republicans wanna raise the voting age back to 21.