Kennedy (D-MA) passed away from brain cancer in August 2009, 15 months after his diagnosis.
There is a greater than 50 percent chance McCain (R-AZ) will pass away at least 1 year from now meaning there will be a special election in November 2018 will take place simultaneously with Flake's seat. Democrats could win both US Senate seats in AZ the same day in November 2018. It will be like the 1994 TN US Senate Elections when Frist-R and Thompson-R got elected. If McCain passes away after the November 2018,The AZ US Senate Special Election could take place in November 2019 or November 2020. Democrats could win the AZ US Senate Special Election in 2019 or 2020. McCain is going to be 81 years old next month, Kennedy was 76 when he was diagnosed with brain cancer.
Democrats could regain control of the US Senate after 2018.
Hold onto every Democratic held seat up in 2018 including the ones in states Trump won-48D and 52R and pick up NV and both AZ US Senate seats. 51D and 49R.

By 2020-DeGette-D would be in the US House for 23 years-She is the most experienced legislator. If Hickenlooper and Perlmutter decide not to run, what are DeGette's chances in winning the primary and general if she decided to run for the US Senate in 2020 against Gardner?

the FBI. The Trump Family got the Bush Family and it's surrogates to defect from the Republican party.

1)FL-Nelson-D -Likely Democratic
2)IN-Donnelly-D- Tossup if Hill,Hollingsworth,Messer,or Rokita-R run. -1D
3)MI-Stabenow-D Solid Democratic.
4)MO-McCaskill-D Tossup if Hawley,Kinder,Luetkemeyer,or Schmitt-R run. -2D
5)MT-Tester-D Lean Democratic if Rosendale or Stapleton-R run.
6)ND-Heitkamp-D Tossup to Lean Democratic if Berg or Cramer-R run.
7)OH-Brown-D Tossup -3D
8)PA-Casey-D Likely Democratic if Barletta or Kelly-R run.
9)WV-Manchin-D Lean Democratic.
10)WI-Baldwin-D Lean Democratic.
Worse case scenario for Democrats is a loss in IN,MO,and OH.
Democrats will pick up AZ and NV. -1D 47D 53R
Democrats have to pick up 4 seats in 2020.
Democrats will hold onto MI,MN,NH,and VA and pick up
1)CO-Hickenlooper,Perlmutter,Kennedy,or Hancock-D
2)ME-if Collins-R retires or loses in the primary.
3)MT-if Bullock-D runs.
4)NC-Foxx or Cowell-D
Democrats need a strong candidate in AK,GA,IA, and KY-if McConnell-R runs again.

US Senate.
2018
AZ-Flake-R/Stanton-D
NV-Heller-R/Rosen-D
2020
CO-Gardner-R/Kennedy-D

Democrats should nominate a top tier candidate to run against for the 2018 AK US House Race against longtime Republican incumbent Donald Young perhaps Mark Begich. If Begich defeats Young in 2018, he could then seek a rematch against Dan Sullivan in the 2020 AK US Senate Election.
Other candidates Democrats could nominate is Ethan Berkowitz and Hollis French.

If Jason Crow wins the Democratic nomination for the 2018 CO US House Race in CD-6 and defeats Coffman in the November General Election, Is there a chance that Jason Crow pull a Jacky Rosen-NV, give up a competitive US House seat after one term to run for the US Senate against the most vulnerable Republican incumbent assuming Hickenlooper and Perlmutter take a pass?

2018
Hold onto every Democratic held seat including the ones in states Trump won by at least a 5 percent margin (IN,MO,MT,ND,OH,and WV) and pick up AZ,NV,and TX.
Likely Scenario in 2018, Democrats lose IN,MO,and OH and pick up AZ and NV. -1D 47D 53R
2020
Hold onto every Democratic held seat including the most vulnerable seats in MI,NH,and VA and pick up CO,GA,IA,KY(if Democrats nominate the right candidate),ME (if Collins retires or loses in the Republican primary to a LePage protégé.),MT (if Bullock runs),NC,and TX (if one of the Castro twins run and Cornyn retires).
Likely Scenario in 2020, Democrats pick up CO,IA,KY,and NC.

2018
AZ-Phil Gordon- D Tossup Democratic against Flake-R/Lean Democratic against Ward-R
NV-Jacky Rosen-D Tossup Democratic against Heller-R/Lean Democratic against Tarkanian-R
TN-Andy Berke-D Likely Republican against Corker-R/Lean Republican against Crim-R
TX-Beto O'Rourke-D Lean Republican against Cruz-R/Tossup Republican against McCaul-R
UT-Jim Matheson-D Likely Republican against Hatch-R/Lean Republican against Romney-R
Democrats will pick up AZ and NV but could lose IN-Donnelly-D and MO-McCaskill-D
2020
Democrats need a net gain of 3 seats to get back in the majority assuming Trump/Pence steal another election.
CO-Michael Hancock-D
GA-Kasim Reed-D
NC-Anthony Foxx-D
TX-Julian Castro-D

Open Seats
ID(Risch-R) Solid Republican- Republican nominee- Labrador or Little -whoever does not win the Republican nomination for Governor in 2018.
IL (Durbin-D)Likely Democratic-Democratic nominee-Bustos. Republican nominee-Roskam,Hultgren,or Kinzinger.
KS (Roberts-R)Likely Republican-Republican nominee-Marshall,Yoder, or Estes. Democratic nominee-Davis or Thompson.
KY (McConnell-R)Lean Republican-Republican nominee-Comer,Guthrie,Massie,or Barr. Democratic nominee-Beshear.
ME (Collins-R)Tossup Democratic-Democratic nominee-Pingree. Republican nominee-Poliquin.
MA (Markey-D)Likely Democratic-Democratic nominee-Kennedy,Clark,or Moulton. Republican nominee-Baker.
MS (Cochran-R)Lean Republican-Republican nominee-Kelly or Palazzo. Democratic nominee-Hood.
NH (Shaheen-D)Tossup Democratic-Democratic nominee-Shea Porter or McLane Kuster. Republican nominee Sununu.
NM (Udall-D)Likely Democratic-Democratic nominee-Balderas. Republican nominee-Sanchez.
OK (Inhofe-R)Solid Republican-Republican nominee-Mullin or Russell. Democratic nominee-Inman
RI (Reed-D)Solid Democratic-Democratic nominee-Cicilline or Langevin. Republican nominee-Morgan
SC (Graham-R-defeated in primary)Lean Republican-Republican nominee-Gowdy. Democratic nominee-Parnell.
TN (Alexander-R)Solid Republican-Republican nominee-Black or Blackburn. Democratic nominee-Dean or Fitzhugh.
TX (Cornyn-R)Lean Republican-Republican nominee-McCaul. Democratic nominee-Castro.
WY (Enzi-R)Solid Republican-Republican nominee-Cheney.
Vulnerable Incumbents
AK-Sullivan-R vs Berkowitz-D Likely Republican
CO-Gardner-R vs Neguse or Kerr-D Tossup Democratic
GA-Perdue-R vs Abrams or Evans-D Lean Republican
IA-Ernst-R vs Boulton or Pritchard-D Lean Republican
MI-Peters-D vs Amash,Bishop,Trott-R Lean Democratic
MN-Franken-D vs Lewis or Paulsen-R Lean Democratic
MT-Daines-R vs Bullock-D Lean Republican
NC-Tillis-R vs Ross-D Tossup Democratic
VA-Warner-D vs Wittman,Taylor,Garrett,Brat,Comstock-R Lean Democratic
Safe Incumbents
AL-Strange-R
AR-Cotton-R
DE-Coons-D
LA-Cassidy-R
NE-Sasse-R
NJ-Booker-D
OR-Merkley-D
SD-Rounds-R
WV-Capito-R

Democrats hold onto vulnerable seats in MI,MN,NH,and VA and pick up CO,NC,and ME-if Collins retires. +3D
If Democrats defeat Flake-AZ and Heller-NV in 2018 and Trump in 2020,Democrats will regain control of the US Senate in 2020.