Assume all other seats are safe for either party besides the toss-up seats below, the Democrats will need to hold:

AL Doug Jones

MI Gary Peters

MN Tina Smith

NH Jeanne Shaheen

NM Tom Udall [*Incumbent not running for re-election in 2020.]

VA Mark Warner

And flip 4 out of the 12 to take over the majority:

AZ Martha McSally

CO Cory Gardner

GA Johnny Isakson [*Incumbent not running for re-election in 2020.]

GA David Perdue

IA Joni Ernst

KS Pat Roberts [*Incumbent not running for re-election in 2020.]

KY Mitch McConnell

ME Susan Collins

MS Cindy Hyde-Smith

MT Steve Daines

NC Thom Tillis

TX John Cornyn

Right now there are 11 Republican seats that are very likely to stay in Republican hands and 6 Democratic seats that are very likely to stay in Democratic hands. This means the Democrats will need to win 3 out of the 12 current Republican seats from the toss-up list above to tie or 4 seats to take over the majority. Of course, we will need to hold all of our toss-ups seats.