US Senate seats up for re-election in 2020 that Democrats will definitely win is-

1)RI(Reed-D)
2)MA(Markey-D)
3)NJ(Booker-D)
4)IL(Durbin-D)
5)DE(Coons-D)
6)OR(Merkley-D)
RI,MA,NJ,IL,DE,and OR are solid Democratic states with Democratic incumbents that won re-election by a double digit margin.
7)VA(Warner-D) performance will be similar to the other VA Democratic US Senator(Kaine-D).
8)NM(Lujan-D) performance will be similar to the other NM Democratic US Senator(Heinrich-D)
9)MN(Smith-D)
10)NH(Shaheen-D)
11)MI(Peters-D)
12)CO(Hickenlooper-D)
MN,NH,MI,and CO are Democratic leaning states at the US Presidential level, Democratic Nominee for the 2020 US Senate races in MN,NH,MI,and CO will benefit from the 2020 Democratic US Presidential Nominee coattails.
13)AZ special(Kelly-D) challenging weak Republican incumbent in swing state(McSally-R)
14)NC(Cunningham-D) challenging weak Republican incumbent in swing state(Tillis-R)
15)ME(Gideon-D) will benefit from the 2020 Democratic US Presidential Nominee's coattails.
16)IA(Greenfield-D) challenging weak Republican incumbent in swing state(Ernst-R)
17)GA special(Tarver-D) challenging weak Republican incumbent in swing state(Loeffler-R)
18)GA regular(Tomlinson-D) challenging weak Republican incumbent in swing state(Perdue-R)