Those saying "no Republican has a chance in 2018," I refer you to 2002. Also, there is no way you'll be able to keep this TDS going for another 2 years. Trump's already being normalized and the constant attacks are going to get stale and you're gonna have to abandon the current platform and actually start offering something other than "Trump is Hitler."
Trump has been giving people plenty to attack him on before and since he took office, and there is no reason to think he won't continue to commit blunder after blunder. That's just how he operates. And if you think all of this won't matter, look at what Republicans did under Obama. Trump isn't as different as you may think. If he was, his approvals wouldn't be sinking. And yes, 2002 was an exception. So was 1998. Both years, though, had unique circumstances and presidents with hugely favorable approvals - like mid-high 60s in many instances. Republicans
could do well in theory, but is it probable given what we've seen? I really doubt it. 2002 was a rare exception in a long line of consistent midterm losses for the "in-party."
Governing is not like campaigning. Eventually supporters do lose faith - or at least enough to cause problems.