Also of note (and just as relevant), Republicans won the Popular vote for Congress by over 3 million.[1]
[1] https://www.cookpolitical.com/charts/house-charts/national-h...
So what does that tell us?
Also historically the last mid terms should have given massive gains in both houses instead of the extremely slim majority in a single house.
The majority of Americans live in cities and have political views which would be considered "leftist" in US parlance - they support Roe V. Wade, gun control, health care reform, etc. That, in practice, the US appears "divided" at all is only due to the outsized influence of the right and rural voters.
https://www.senate.gov/about/origins-foundations/senate-and-...
The overall final compromise did include compromises to slave owning states, but this vestigial is not due to those compromises. In fact, they were against it.