If you could change the result of one senate race in favor of both parties in each election cycle

[OSR stands with Israel]

And you can only choose races that were decided by a 15 points or less. This is my list from 2008 onwards

2008: OR and GA
2010: WA and KY
2012: MA and AZ
2014: VA and GA
2016: NH and IN
2018: NV and TN
2020: CO and MS
2022: NV and OH

[GAinDC]

2008: Georgia
2010: Wisconsin
2012: Nevada
2014: Colorado
2016: Pennsylvania
2018: Indiana
2020: Maine
2022: North Carolina

[President of the great nation of 🏳️‍⚧️]

2008: MN and KY
2010: WA and WI (sorry Patty Murray)
2012: ND and AZ (not really enthusiastic about either)
2014: NH and NC (I think it'd be funny to have a two-state Senator in the present day)
2016: NH and WI
2018: AZ and MO (and what the hell; WV and IN too)
2020: CO and NC (f[inks] Cunningham, but I still want a Democrat from NC, and Gardner would be blanched in 2026)
2022: PA and WI

I guess senators in the seats I changed ITTL:
MN-2:
Norm Coleman (2003-2015)
Betty McCollum (2015-)
KY-2:
Mitch McConnell (1985-2009)
Bruce Lunsford (2009-2015)
Matt Bevin (2015-)
WA-3:
Patty Murray (1993-2011)
Dino Rossi (2011-2017)
Suzan DelBene (2017-)
WI-3:
Russ Feingold (1993-)
ND-1:
Kent Conrad (1992-2013)
Rick Berg (2013-)
AZ-1:
Jon Kyl (1995-2013)
Richard Carmona (2013-2019)
Martha McSally (2019-, though I'd prefer a re-elected Carmona to McSally)
AZ-3 (changed through butterflies of McSally winning):
John McCain (1987-2018)
Jon Kyl (2018-2020)
Mark Kelly (2020-)
NH-2:
Jeanne Shaheen (2009-2015)
Scott Brown (2015-2021)
Annie Kuster (2021-)
NC-2:
Kay Hagan (2009-)
NH-3:
Kelly Ayotte (2011-2023)
Joyce Craig (2023-)
MO-1:
Claire McCaskill (2007-)
CO-2:
Cory Gardner (2015-)
PA-3:
Pat Toomey (2011-2023)
Mehmet Oz (2023-)

Honorable mention:
WV-1:
Joe Manchin (2010-2019)
Patrick Morrisey (2019-)
IN-1:
Joe Donnelly (2013-)

EDIT: Considering trends, Georgia would've been a good pick in 2008 as well; I just wanted the satisfaction of taking down Cocaine Mitch.

[America Needs a 13-6 Progressive SCOTUS]

I am treating each of these as their own scenario

2008: NJ and KY
NJ because its the only race that year Dems won by less than 15% where Dems would probably have gotten the seat back in 2014, and KY because getting rid of McConnell weakens R obstruction, and there were no races Reps won by less than 15% where Dems could probably hold on in 2014
2010: WA and WI
WA because Dems definitely get it back in 2016, and WI because it probably allows Dems to hold on in 2016 and 2022, and Feingold was also really good
2012: ND and AZ
ND because Dems lost it in 2018 anyways, and Heitkamp was never all that good anyways, and AZ because it prevents Sinema who was a bad Dems from winning in 2018
2014: MN and NC
MN because Franken was a sexual predator and in a seat Dems probably could have taken back in 2020, and NC because it probably saves Hagans life and Dems probably could have held on in 2020
2016: IL and WI
IL because Dems would have gotten it back in 2022, and Kirk probably would have placed a meaningful level of votes against his party in the process too, and WI has the same logic as with 2010
2018: WV and FL
WV because Manchin was a bad Dem in a seat Dems would lose in 2024 no matter what anyways, and FL because Rick Scott is that terrible
2020: VA and ME
VA because Dems probably get that seat back in 2026 no matter what, and ME because it probably shores up Dems in being able to hold the seat no matter what in 2026
2022: CO and WI
CO because Dems probably get that seat back in 2028 no matter what, against a Rep who was a bit less bad than most, and WI because it probably matters the most going into 2028

[Tekken_Guy]

I am treating each of these as their own scenario

2008: NJ and KY
NJ because its the only race that year Dems won by less than 15% where Dems would probably have gotten the seat back in 2014, and KY because getting rid of McConnell weakens R obstruction, and there were no races Reps won by less than 15% where Dems could probably hold on in 2014
2010: WA and WI
WA because Dems definitely get it back in 2016, and WI because it probably allows Dems to hold on in 2016 and 2022, and Feingold was also really good
2012: ND and AZ
ND because Dems lost it in 2018 anyways, and Heitkamp was never all that good anyways, and AZ because it prevents Sinema who was a bad Dems from winning in 2018
2014: MN and NC
MN because Franken was a sexual predator and in a seat Dems probably could have taken back in 2020, and NC because it probably saves Hagans life and Dems probably could have held on in 2020
2016: IL and WI
IL because Dems would have gotten it back in 2022, and Kirk probably would have placed a meaningful level of votes against his party in the process too, and WI has the same logic as with 2010
2018: WV and FL
WV because Manchin was a bad Dem in a seat Dems would lose in 2024 no matter what anyways, and FL because Rick Scott is that terrible
2020: VA and ME
VA because Dems probably get that seat back in 2026 no matter what, and ME because it probably shores up Dems in being able to hold the seat no matter what in 2026
2022: CO and WI
CO because Dems probably get that seat back in 2028 no matter what, against a Rep who was a bit less bad than most, and WI because it probably matters the most going into 2028

If you want to get rid of Sinema why not have her lose in 2018?

[Spectator]

2008: MN and KY—super easy
2010: CA and WV
2012: MA and AZ
2014: AK and VA
2016: NH and MO
2018: AZ and FL
2020: CO and SC
2022: god all the Republicans sucked that year but I guess CO for WI

[Ferguson97]

Assuming each cycle is in a vacuum (ie, I can choose Susan Collins in both 2008 and 2020):

2008: North Carolina (she loses in 2014 anyway) and Kentucky (bye-bye, Mitch)
2010: Washington (fairly easy to win back in 2016) and Wisconsin (bye-bye, Ron Johnson)
2012: North Dakota (she loses in 2017 anyway) and Arizona (butterflies away Sinema)
2014: Virginia (easier to flip back than NH) and North Carolina (harder for Republicans to flip than any of the other options)
2016: Nevada (probably easiest to flip back) and Missouri (I like Jason Kander)
2018: West Virginia (Manchin retires in 2024 anyway) and Texas (easiest choice of my life)
2020: New Mexico (should be easy to flip back in 2026, even in a Biden midterm) and Maine (since I can't pick Doug Jones)
2022: Nevada (solely because Laxalt is the least insane of all the options) and Ohio (JD Vance is a cretin)

[OSR stands with Israel]

Assuming each cycle is in a vacuum (ie, I can choose Susan Collins in both 2008 and 2020):

2008: North Carolina (she loses in 2014 anyway) and Kentucky (bye-bye, Mitch)
2010: Washington (fairly easy to win back in 2016) and Wisconsin (bye-bye, Ron Johnson)
2012: North Dakota (she loses in 2017 anyway) and Arizona (butterflies away Sinema)
2014: Virginia (easier to flip back than NH) and North Carolina (harder for Republicans to flip than any of the other options)
2016: Nevada (probably easiest to flip back) and Missouri (I like Jason Kander)
2018: West Virginia (Manchin retires in 2024 anyway) and Texas (easiest choice of my life)
2020: New Mexico (should be easy to flip back in 2026, even in a Biden midterm) and Maine (since I can't pick Doug Jones)
2022: Nevada (solely because Laxalt is the least insane of all the options) and Ohio (JD Vance is a cretin)

Surprised you didn’t go for O’Dea in 2022 given he lost by less than 15 and it would be an easier flip for the Dems in 28 than NV

[Heisenberg]

Fun thread!
Assuming each is in a vacuum (hence, getting rid of Graham all three times), here's mine

2008: SC (bye bye, Lindsey and easy to win back in 2014) and MN (I think Republicans hold it in 2014 but Coleman sees the writing on the wall and retires in 2020)
2010: IL and CO (I wish I could say DE with Mike Castle, because he holds the seat in 2014 as well but he didn't win the primary IRL and the margin for the OTL general was >15%. However, in 2016, Glenn came within 5 of Bennet despite Trump losing the state, so maybe Buck could hold on? He's toast in 2022 regardless)
2012: AZ (Flake wasn't that good and this seat would fall the next cycle anyway) and MT (getting rid of Tester)
2014: SC and MI (If I can't do SC again, then CO because Gardner is toast in 2020 anyways)
2016: AZ (Republicans would lose this seat before 2022 anyway) and NV
2018: MS (I guess, because it's up two years later and should be won back) and OH
2020: SC (If I have to pick someone else, uh? IA? Probably the easiest to win back) and GA (The Perdue seat)
2022: IA and NV: These were hard, but Iowa should be (on paper) the easiest to win back in 2028 and Laxalt is probably my favorite of the unsuccessful GOP nominees (honestly, most of them SUCKED). Of course, I'm treating each of these years in a vacuum. If we're pretending Heck (whose race I flipped in 2016 in this scenario) is up for re-election, then AZ, I guess.

[Mr. Smith]

2006: TN [Ford would be an awesome to 2018 keeper] and NJ [f&*k Menendez]
2008: MS [Musgrove could Doug Jones the place up for a time] and LA [sorry Landrieu]
2010: WI [keep Feingold] and CO [sorry Bennet]
2012: AZ [don't want Flake] and ND [Heitkamp is borrowed time anyway]
2014: NC [keep Hagan, she was supposed to win anyway] and MI [because f*(k Peters]
2016: KY [rare opportunity to dump Paul] and NH [literally just out of spite for a certain meme]
2018: TX [because f*&k Cruz] and NJ  [because f*&k Menendez]
2020: MT [Bullock is too cool to not have] and MI [Peters is a sellout anyway]
2022: WI [f*(k RoJo] and CO [sorry Bennet, O'Dea is the sanest]

[Vosem]

Assuming each cycle is a vacuum like Ferguson does:

2008: Minnesota (Coleman was a very good Senator) and Mississippi (probably the most conservative Democrat who came within 15%; South Carolina is just out at 15.2%)
2010: Washington (Rossi had rotten luck) and Indiana (electing Coats was useless, Democrats couldn't have held it)
2012: Massachusetts (the backlash to Obamacare must be represented forever) and Arizona (Flake was also useless)
2014: New Hampshire (the backlash to Obamacare must be represented forever) and Georgia (...Perdue was also useless)
2016: New Hampshire (Ayotte was also a very good Senator; Illinois is outside your 15% threshold) and Indiana (not a fan of Young)
2018: Arizona (McSally had promise) and Missouri (Hawley can go screw himself)
2020: Arizona (McSally had promise) and South Carolina (Graham is a relic of another age at this point)
2022: Arizona (Masters had promise; this feels uninspired because so many Democrats won by just a little short of 15%, though) and Ohio (Vance can go screw himself)

[Kabam]

Not counting special elections and pretending everything happens in a vacuum.

2008: Kentucky (They will flip all in 2014 anyway, so bye Mitch!) and North Carolina (Will flip in 2014 anyway. Dems are in a better position in 2020 with an open seat or an 80+ Dole. And the butterfly effect might prevent Hagan's death.)
2010: Wisconsin (Bye, Ron Johnson. Better chances to win in 2016 and 2022.) and California (Boxer retires in 2016 anyway and automatic flip in 2016.)
2012: Arizona (Carmona is probably better than both Sinema and Berkely) and Florida (Flips in 2018 anyway. Maybe Dems can find a better candidate there.)
2014: Alaska (Given Peltola's recent success, Begich has good chances to hold on in 2020. And it prevents Sullivan, who is hard to defeat.) and Illinois (Easy flip in 2020 and someone younger there might be nice.)
2016: Wisconsin (Bye, Ron Johnson [2] and really the only good option for Dems) and Colorado (By far easiest to win back in 2022.)
2018: Texas (Best chances to keep in 2024 and Beto would be a good senator) and West Virginia (Gone in 2024 anyway, with Manchin retiring. Menendez in NJ (the other good option) is a more solid vote for Dems.)
2020: Texas (Probably not much harder to keep in 2026 than North Carolina. Hegar > Cunningham and Tillis might retire anyway, otherwise he is easier to beat than Cornyn. Also thinking Collins will retire anyway.) and Colorado (Should be easier to flip in 2026 than others and Hickenlooper does not really seem to want to be in the Senate anyway.)
2022: North Carolina (Very hard between WI and NC. But Beasley might have better chances to hold on in 2028. And I prefer her over Barnes.) and Connecticut (Blumenthal is getting old and defeating Levy would be so easy in 2028.)

[DS0816]

And you can only choose races that were decided by a 15 points or less. …

2022 U.S. Senate in Pennsylvania.

[Amenhotep Bakari-Sellers]

Obviously, Vance v Rush Ryan should of been elected

[Lambsbread]

2018 I'd trade Sinema for Claire McCaskill, Bill Nelson, or Beto.