The quoted material below is Charlie Cook’s current (June 12th) Senate summary.
I will ignore the SOLID seats for both parties and deal with the rest.
First of all, the Democrats. Cook lists 4 seats in play. Alabama is pretty simple. Tuberville will likely defeat Sessions in the Republican primary, but either one will easily dispose of Jones in the General Election. I believe Peters will retain his Senate seat in Michigan and that it won’t be close. Right now, Tina Smith looks in good shape to hold Minnesota’s seat and no Republican has even distinguished himself in this race yet. As for New Mexico, this seat should be listed as SAFE D, rather than LIKELY D. The uncontested Democratic Senate primary had more voters than the contested Republican primary, which should be the first clue.
Democrats will likely lose Alabama and will not be in any serious jeopardy elsewhere.
As for the Republicans, it starts getting ugly real fast.
Arizona - McSally is a dead women walking. This race was over before it began.
Colorado - The “slave ship” gaffe and ethics controversy, both against Hickenlooper, will not save Gardner. Gardner is pretty much toast.
Maine - Collins is not doing well. Democrats have recruited a couple of good candidates and I think Collins is gone.
North Carolina - This is going to be a battle royale and will be extremely close regardless of who wins. But Tillis is in major jeopardy. Republicans in the suburbs of Charlotte could help put Cunningham over the top. Right now, I would tilt this to Cunningham.
Georgia - Democrats could sweep both Senate seats, although I think Perdue is in stronger shape. Loeffler is toast in the Republican primary. Democrats have done a good job at voter registration and an excellent job at turnout in the primary, amazing considering how totally cluster ****** the election was. Republicans or Democrats could win the trifecta or the elections could be split. Right now, Trump/Perdue/Collins win by the skin of their asses. I should note that the Democrats nominated a strong candidate against Perdue, which should make things all the closer.
Iowa - I believe Ernst is in big trouble. Greenfield has pulled ahead in polling and remember that Iowa sent liberal lion Tom Harkin to the Senate for years and this is Harkin’s old seat. Suburban Republicans could very well send Greenfield to the Senate. I will tilt this race to Greenfield.
Kansas - I believe a Republican will win this open seat, but it is going to be a fight in a State where Republicans should expect to cruise to victory. Again, suburban Republicans will likely be defecting to some degree.
Montana - Democrats have recruited a strong candidate and have a lengthy history of winning here. I am tilting this to Democrat for the time being.
Alaska - Sullivan is weak enough that this race was recently downgraded from SAFE R to LIKELY R. I think Sullivan wins, however.
Kentucky - There clearly will be a hard fought contest here between McGrath and McConnell. I think McConnell will prevail, but he has a fight on his hands.
South Carolina - Graham is doing very anemically, but will likely be saved by the overall Republican tilt of the State.
Texas - Texas is in play, but Cornyn wins. Democrats are likely to have a number of victories in Texas, but not in this particular race.
(Note: Texas is very close to going purple, but we are NOT quite at that point yet. Possible the next election cycle, more likely the subsequent election cycle. However, I would not bet against the Democrats taking the Texas House of Representatives this year.)
Republican raw gain - 1 seat.
Democratic raw gain - 6 seats
Democratic net gain - 5 seats
That would shift the Senate from the current 53 to 47 Republican majority to a 52 to 48 Democratic majority.
The Republican best case scenario is a net shift of D+2.
The Republican worst case scenario is a net shift of D+12, though that would be extremely unlikely. A Democratic landslide nationwide would likely produce a net shift of D+7
I think that, overall, it will be bad but not catastrophic for Republicans. The result will almost certainly be a Democratic Senate, but not a heavily Democratic Senate and with enough swing State Democrats to limit their legislative potential.
A Senate that would still likely tone down whatever comes out of the House of Representatives.
If Biden wins the presidency and a narrow Senate, certainly not a catastrophic situation for Republicans. Biden would be far weaker in 2021 and 2022 than Obama was in 2009 and 2010.
DEMOCRATS | 12 HELD SEATS
REPUBLICANS | 23 HELD SEATS