In the 2018 House selections the situation was that the RP did very little to safeguard seats held by them and concentrated more on solidifying their hold on the Senate, Because it does not result in real change in governance, there is always a lower turn-out in mid-term elections are much lower than it is in the presidential election.
For instance in the 2016 election circa 130 million voters voted - in the House elections in 2018 only circa 90 million people voted. It was for that reason the DP won seats they would not normally have won. The fact is that midterm results could be changed in presidential election year - but they often did not change depending on who the presidential candidate is. Even though the RP got House control in 2014 - they retain control in 2016.
In a recent poll 68% of the voters indicate that the House is doing an atrocious job and that percentage is even increasing, That could (like happened in England) resulted in seats in the Democratic heartlands turn conservative in 2020 and it coud do havoc to DP representation in the House,