I, like many other people, have interpreted Trump's involvement in the Republican primaries as increasing his power in the party at the cost of reducing the party's power in Congress, pushing candidates who were loyal to him but less electable than their rivals. We might be wrong. Most people badly underestimated Trump's ability to get votes in 2016. Perhaps we are underestimating the attraction of his political style this time as well.
There is a way to find out.
Make a list of Republican candidates who won their primaries with Trump's backing. Estimate what the midterm vote should be for every seat in Congress based on data from previous elections, taking account of location, incumbent advantage, and whatever else you need to fit the data. After the election, see if the Trump candidates did better or worse relative to the estimate than the non-Trump candidates.
Ideally, the first two parts should be done before the election results are known, but it's probably too late for that unless someone is already doing it.