I think Bill Kristol has the right of it:
One can argue Republicans didn’t really do that well given the strong economy. One can argue Trump is on net a burden to the party and that in the long term he’ll do great damage. But one must also admit the public did not rise up as one yesterday to repudiate Trump and Trumpism.
Of course it didn’t take long for Trump, emboldened by Republican gains in the Senate, to start the process of dismantling the Mueller investigation by firing* Sessions and replacing him with a loyalist. (* Sorry: I meant “accepting the resignation of”.)