In the mid=term election in 2018 about 92 000 000 votes were cast in the election. Bearing in mind that in the 2016 election there were more or less 126 000 000 votes casted. That means that mire of less 34 000 000 voters did not vote in the mid=term election
It is fairly normal that the Party holding the Presidency lose seats in the mid-term election and it happens fairly regularly. The reason is that the minority party works very hard while the leading party is more complacent.
Senate
In the 2018 election the Republicans were more worried about what will happen in the Senate and concentrated more on the Senate Races where the Republicans held 52 seats and the Democrats 48.
In the Senatorial elections the Republicans gained 4 seats previously held by the Democrats and lost two seats previously held by Republicans. So in the end there are 54 seats held by the Republicans and 46 held by Democrats
In 2020 there are 33 seats up for election of which the parties hold the following seats:-
Republicans - 22
Democrats - 11
Of the above seats 19 Republican Seats are strongly republican and 3 are in States that voted republican with large majorities of between 5 and 10%.
Of the 11 Democrat seats 3 are in states won by Trump in 2016 and one is marginal at best.
That means that the Republicans can gain up to 4 seats in the Senate in 2020 and retain all the seats they ho;d at present.
House of Representatives
In the 2018 election the Democratic Party won in 35 seats held previously held by Republicans. Of these about half of t hose seats were won by margins of less than 8 000 votes and is marginal. If the result is seen in the context of the noprmal General election turnout to be expected the chances are that there will be a swing to the Republican party in at least 28 of the seats the Democrats won in 2018.