The last time Democrats had regained control of the US Senate was in 2006.

Democrats won every Democratic held US Senate seat up in 2006 and picked up
PA(Casey-D vs Santorum-R)
OH(Brown-D vs DeWine-R)
RI(Whitehouse-D vs Chaffee-R)
MO(McCaskill-D vs Talent-R)
MT(Tester-D vs Burns-R)
VA(Webb-D vs Allen-R)

Democrats guide to regaining control of the US Senate in 2020 is winning every Democratic held US Senate seat up in 2020 except AL(Jones-D) and pick up
CO(Hickenlooper-D vs Gardner-R)
AZ special(Kelly-D vs McSally-R)
NC(Cunningham-D vs Tillis-R)
ME(Gideon-D vs Collins-R)
IA(Greenfield-D vs Ernst-R)