The states are too well gerrymandered for the house to swing democratic.
Gerrymandering is a double edged sword. All things being equal it provides you a solid majority in representation with only a tiny majority (or plurality) of votes. The stronger the gerrymandering the greater the danger of massive swings against you from only marginal shifts in the opinion of the electorate.
The thing is, with Trump at the helm, everything will not be equal. He is absolutely reviled among Democrats, ensuring a high level of voter participation from their base. He is alienating many Republican supporters and panders mostly to the deplorable vote, which could well drive down Republican voter participation. Other than loads of empty promises and hot air, he will not deliver much in the next two years.
Granted, deplorables will still vote for him, but if he manages to drive down overall Republican participation somewhat and drive up Democrat participation and have independents break against him, there could well be a blue wave in 2018.
Think of it this way, Republican gerrymandering has made the seats Democrats do hold secure. Republicans will not be able to seriously threaten them in their home constituencies, even if they become significantly more popular than they are. On the other hand, comparatively minor swings of the electorate could result in massive seat swings towards Democrats.
There is also the demographic shift, which favors the Democrats. Two years isn't very long for this factor, but this is in addition to all the other factors mentioned already.
McHrozni