Who would have been the strongest primary challenger against Obama in 2012?

[VPH]

I've been reading a bit about the 2012 campaign and find it surprising that there was actually discussion about a primary challenge to President Barack Obama. Bernie Sanders discussed the idea at the time. Harold Ford was even mentioned, weirdly enough?
https://www.npr.org/templates/story/story.php?storyId=125662980
https://newrepublic.com/article/79530/democratic-primary-challenge-obama-2012

I don't think anybody would find much success in doing so, but who would have been the most formidable candidate for a primary challenge and what would they have maxed out at?

[Gracile]

There is no way Obama would have lost the Democratic primary no matter how “strong” an opponent he would garner.

[brucejoel99]

If he hadn't swam in the birtherism fever swamp, or already re-registered back as a Republican that year after a brief stint as an Independent, or even made speeches at CPAC endorsing Mitt Romney, then Trump. He's got the ego & was a Democrat for quite a long time.

[Orser67]

Assuming that she planned ahead for the run, I think that Clinton would have been his most formidable challenger, though even she would have stood little chance of winning.

More realistically, Sanders is probably the best bet for running a semi-competitive race.

[538Electoral]

None.