In examining the possible causes of your confusion, it is important to understand some key factors
- All 435 seats for the House of Representatives were up for re-election for the midterms. Key point: these seats are up for election every cycle.
- Only 1/3 of the Senate was up for re-election this midterm cycle. These seats are only voted on every 6 years.
- There were a few special elections for the Senate this Year.
- While Senators are elected State wide, Representatives are limited to their voting District.
While it is possible some people split their ticket, voting for both Democrats and Republicans, it is more likely that the sum of votes for Democrat Representatives in a State would equal the votes for the Democrat Senator (in States where a Senator was up for re-election).
As an example, consider Missouri. Claire McCaskill, a Democratic Senator, was up for re-election this cycle in a State won by Trump in 2016 (57% to 38%). Missouri has 8 seats in the House of Representatives.
Senate Results
- Hawley: 1,245,732
- McCaskill: 1,101,377
- Other: 73,720
Total House Votes
- Republican: 1,318,481
- Democrat: 1,016,095
- Other: 58,793
Differences
- 27,460 MORE Votes were cast for the Senate Seat
- Hawley underperformed relative to the House Votes
- McCaskill overperformed relative to the House Votes
As a counterexample, consider West Virginia, and Joe Manchin. Another Democratic Senator up for re-election in a state won by Trump (68% to 26%). West Virginia has 3 House Seats.
Senate Results
- Morrisey: 269,872
- Manchin: 288,808
- Other: 24,231
Total House Votes
- Republican: 335,791
- Democrat: 232,856
- Other: 6,213
Differences
- 8,051 MORE Votes were cast for the Senate Seat
- Morrisey underperformed relative to the House Votes
- Manchin GREATLY overperformed relative to the House Votes
What could be a contributing factor in the different outcomes between these two results? Manchin voted for Supreme Court Justice Kavanaugh's confirmation, while McCaskill voted against it. Yet, even this isn't a full proof indication. Turn to Montana, with Democratic Senator Tester up for re-election. Fortunately for the math, Montana only has one seat in the house.
Senate Results
- Rosendale: 223,687
- Tester: 237,986
- Other: 13,563
Total House Votes
- Gianforte: 243,436
- Williams: 218,635
- Other: 13,373
Differences
- 208 fewer Votes were cast for the Senate Seat
Here, it appears to be strength of the incumbents that held Tester through the election. Similar analysis could be performed for the other 30 states that had a Senator on the ballot this year.