The party in power always loses midterms. The question is how much damage. They are motivated and we should never underestimate that.
In 2002, when GWB was midway through his first term, the GOP picked up 2 senate seats and 8 house seats.
The dynamic is a little different this year in that Trump is seen by many as a "people's" president as opposed to a partisan Republican president. If the economy continues to surge, the same blue collar Democrats from places like PA, MI, WI, OH and IA that helped elect him in 2016 will come out in support of his agenda in 2018 mid-terms.
The Democrat party is quickly becoming the party of cranks, weirdos and haters of America.