Midterm elections are almost always bad for the party in power, with I believe 30 House seats lost on average. 2018 is likely to be no different, particularly with a large anti-Trump undertow. However, unlike the 2010 Tea Party massacre in the wake of extreme federal overreach by the Obama Administration, the current form of the political Left has next to no policy platform from which to oppose the evil Trump Administration.
There are too many “jump ball” factors to make a high probability prediction at this point, but I believe senior advisor to the POTUS and expert pollster, KellyAnne Conway, is spot on with the following commentary (source: Axios):