If these races went differently...

[President Punxsutawney Phil]

KY-SEN 2004: Mongiardo wins by 2%
SD-SEN 2004: Daschle wins by 3%
LA-SEN 2004: Vitter gets 47% in first round, loses 51-49% to Chris John in runoff
NC-SEN 2004: Bowles wins by 0.5%
TN-SEN 2006: Ford wins by 1%
MN-SEN 2008: Franken wins by 2%, so he is thus seated on schedule
KY-SEN 2008: Lunsford wins by 1%
WI-SEN 2010: Feingold wins by 2%
PA-SEN 2010: Sestak wins by 2%

What happens afterwards?

[Calthrina950]

KY-SEN 2004: Mongiardo wins by 2%
SD-SEN 2004: Daschle wins by 3%
LA-SEN 2004: Vitter gets 47% in first round, loses 51-49% to Chris John in runoff
NC-SEN 2004: Bowles wins by 0.5%
TN-SEN 2006: Ford wins by 1%
MN-SEN 2008: Franken wins by 2%, so he is thus seated on schedule
KY-SEN 2008: Lunsford wins by 1%
WI-SEN 2010: Feingold wins by 2%
PA-SEN 2010: Sestak wins by 2%

What happens afterwards?

I can guess on a few of these. I imagine that if Ford had won in 2006, and assuming everything else went the same, he would have been extremely vulnerable in 2012. I don't see much change with Franken, but Lunsford's victory would have major implications for political developments over the last ten years. Feingold and Sestak victories would suggest that the Republican wave of 2010 would be much more limited in impact than it actually was.

[DevinM626]

SD-SEN 2004: Daschle wins by 3%
WI-SEN 2010: Feingold wins by 2%

What happens afterwards?

Daschle likely loses in 2010, especially if the Republican Wave is as big as it was in real life.

Feingold would still be a Senator today, as well as a potential 2020 candidate (whether he would go for it, who knows).

[nerd73]

SD-SEN 2004: Daschle wins by 3%
WI-SEN 2010: Feingold wins by 2%

What happens afterwards?

Daschle likely loses in 2010, especially if the Republican Wave is as big as it was in real life.

Feingold would still be a Senator today, as well as a potential 2020 candidate (whether he would go for it, who knows).

Two Senate majority/minority leaders losing re-election back-to-back, has that happened before?

[President Punxsutawney Phil]

SD-SEN 2004: Daschle wins by 3%
WI-SEN 2010: Feingold wins by 2%

What happens afterwards?

Daschle likely loses in 2010, especially if the Republican Wave is as big as it was in real life.

Feingold would still be a Senator today, as well as a potential 2020 candidate (whether he would go for it, who knows).

Two Senate majority/minority leaders losing re-election back-to-back, has that happened before?

In both 1950 and 1952, the Democratic Senate leader lost his seat. It led some to actually think the job was cursed.

[UWS]

KY-SEN 2004: Mongiardo wins by 2%
SD-SEN 2004: Daschle wins by 3%
LA-SEN 2004: Vitter gets 47% in first round, loses 51-49% to Chris John in runoff
NC-SEN 2004: Bowles wins by 0.5%
TN-SEN 2006: Ford wins by 1%
MN-SEN 2008: Franken wins by 2%, so he is thus seated on schedule
KY-SEN 2008: Lunsford wins by 1%
WI-SEN 2010: Feingold wins by 2%
PA-SEN 2010: Sestak wins by 2%

What happens afterwards?

That means that the Democrats would already have won the Senate majority in 2006 instead of falling short with 49 seats for each of the two major parties.

For Kentucky, if Lunsford defeated McConnell in 2008, I guess John Cornyn would have succeeded him as Senate Republican Leader.

[President Punxsutawney Phil]

KY-SEN 2004: Mongiardo wins by 2%
SD-SEN 2004: Daschle wins by 3%
LA-SEN 2004: Vitter gets 47% in first round, loses 51-49% to Chris John in runoff
NC-SEN 2004: Bowles wins by 0.5%
TN-SEN 2006: Ford wins by 1%
MN-SEN 2008: Franken wins by 2%, so he is thus seated on schedule
KY-SEN 2008: Lunsford wins by 1%
WI-SEN 2010: Feingold wins by 2%
PA-SEN 2010: Sestak wins by 2%

What happens afterwards?

That means that the Democrats would already have won the Senate majority in 2006 instead of falling short with 49 seats for each of the two major parties.

For Kentucky, if Lunsford defeated McConnell in 2008, I guess John Cornyn would have succeeded him as Senate Republican Leader.

How does Bush's last two years in office change?

[Politician]

KY-SEN 2004: Mongiardo wins by 2%
SD-SEN 2004: Daschle wins by 3%
LA-SEN 2004: Vitter gets 47% in first round, loses 51-49% to Chris John in runoff
NC-SEN 2004: Bowles wins by 0.5%
TN-SEN 2006: Ford wins by 1%
MN-SEN 2008: Franken wins by 2%, so he is thus seated on schedule
KY-SEN 2008: Lunsford wins by 1%
WI-SEN 2010: Feingold wins by 2%
PA-SEN 2010: Sestak wins by 2%

What happens afterwards?

For Sestak and Feingold, they would still be senators even if Trump won in 2016, and would be possible presidential contenders.

For 2008, if McConnell lost Cornyn or Thune would become the next majority leader. It would also be a big blow to Republicans to lose their own leader.

For 2006, if Ford won in 2006 he likely would have lost in 2012 provided his opponent wasn't an Akin/Mourdock/Angle.

For 2004, it would make it much easier for Democrats to win the Senate in 2006 and they would have had a supermajority even after Scott Brown won the special election (assuming Specter still switches parties).

Some other ones

IL-2010: Giannoulias wins
GA-2008: Martin takes down Chambliss
MS-2008 SPECIAL: Musgrove takes out Wicker
FL-SEN 2004: Castor wins