If Trump were to carry 48 states in 2020 like Reagan did in 1984,
The 2020 US Senate Election cycle will be similar to the 1984 US Senate Election cycle when Democrats had a net gain of 2 seats in the US Senate.
During the 1984 US Senate Election cycle:
1 Democratic incumbent US Senator lost re-election by a less than .5 percent margin- Huddleston-KY lost to the Turtle.
2 Republican incumbent US Senators lost re-election.-
IL- Percy-R lost to Simon-D by a 1.9 percent margin.
IA- Jepsen-R lost to Harkin-D by a 11.8 percent margin.
Democrats also won the open Republican held seat in TN formerly held by Baker-R, Gore-D won that Election by a 26.9 percent margin.
Reagan carried IL by a 12.9 percent margin, IA by a 7.4 percent margin, KY by a 20.7 percent margin and TN by a 16.3 percent margin.
In 2020, Democrats will lose AL(Jones-D) by a less than 1 percent margin and pick up AZ(OPEN-Kyl-R),CO(Gardner-R),ME(Collins-R)and NC(Tillis-R) by a wider margin. Trump is likely to win those states(AZ,CO,ME,and NC-) by a less than 5 percent margin and AL by a greater than 20 percent margin.