Barack Obama's career if he lost the 2008 nomination

[Sir Mohamed]

Assuming the 2008 primaries are still close, Obama would probably get a place in Clinton's cabinet. Something like Secretary of Labor, which he would be until running for president in 2016. Whether or not he wins depends on who the Republicans nominate and if he's able to keep up enthusiasm.

That's reasonable, but AG would be more fitting and for Obama. Subsequently he'd step down in early 2015 to focus on his 2016 WH bid and as the OP already concluded defeated Mr. Trump in the GE with the 278 freiwall map + NC.

That said, I find the scenario of Obama retiring from the senate in 2010 and running 4 gov far from impossible. He'd be a far more experienced prez after taking office in 2017, but he'd (most likely) have less room for getting big legislation done since the GOP would at least control the House. Like HRC would have been for him in our TL, Obama as Clinton successor/45th POTUS would have been left to preserve HRC's legacy by governing through executive actions and vetoing GOP bills to dismantle her policies.

[Sir Mohamed]

I honestly think that Obama would just run for re-election to the senate in 2010 to avoid a competitive primary with Pat Quinn. He’d still have a high national profile and would easily be the frontrunner for president in 2016. I imagine whoever Clinton picked for VP (Kaine, Vilsack, Richardson, etc) would not run in 2016 if Obama was plotting a bid as I would assume they would have the sense to know they couldn’t beat him.

Ugh, a gov-primary against Quinn wouldn't be competitive. Quinn wouldn't even be the incumbent because Blago was just removed from office as a result of Obama leaving the senate early. But even if Quinn is the incumbent gov here, it doesn't change much. He almost lost the (real) 2010 Dem primary against the State Comptroller. If he could only beat this dude by less than 1 pt (see: https://en.wikipedia.org/wiki/2010_Illinois_gubernatorial_election#Results), Obama would have won in a cakewalk.

[Sir Mohamed]

If Obama lost the nomination, Hillary almost certainly would have picked him as VP. She'd lost a lot of support amongst African-American voters and picking Obama would have helped restore that for the general election (compared to the IRL scenario, where Obama won most women voters, many African-American men could easily defect to McCain with Obama's defeat, even as African-American women stick with Clinton due to the historic nature of her candidacy.) After Hillary wins reelection in 2012 (I think she, like Obama defeats Romney in this scenario, possibly by an even bigger margin), Obama runs for President again in 2016, and faces Trump. It would be a tossup race, but Obama is the perfect candidate to take on Trump (compared to how Hillary did IRL), and thus he wins. With COVID, he loses in 2020, likely to Pence, or possibly Paul Ryan.

Why wouldn't Obama get reelected in 2020? Sure, running for a 4th consecutive Dem term would be tricky, but COVID may actually have saved him from losing the election. Out of question he would have been far more competent in handling the crisis with a team of professionals. If he played his cards right, he would have won reelection. The GOP may actually be forced to nominate a more reasonable nominee in 2024 after losing with Trump in 2016 and Pence in 2020.