Midterms also tend to have better R than D turnout.
10 of the seats D's have to defend are in states Trump just won. Another half dozen are in states that R's have managed a statewide win recently for either Senate or Governor.
Only Nevada is at risk on the R side.
If R's don't screw up, they should be able to get to 60 Senate seats. If they do well, they could get into the mid-60's.