Special elections since Trump:

Kansas: 2016 won by R by 31 points. 2017: Won by R by 7 points. Swing +24% D.
Montana : 2016 won by R by 15 points. 2017: Won by R by 6 points. Swing +9% D.
California: only D candidates ran in 2017 S.E.
Georgia: 2016 won by R by 23 points. 2017: Won by R by 3 points. Swing +20% D.
SC: 2016 won by R 21 points. 2017: Won by R by 3 points. Swing +18% D.
Utah: 2016 won by R 47 points. 2017: Won by R 32 points. Swing +15% D.
PA: 2016 won by R 20 points. 2017: tossup (0 points). Swing +20% D.

See a pattern there?