Sitting VP nominated again.

[Kingpoleon]

I could see it happening. Say Bush chose Christine Whitman in 2000. Whitman runs in 2008, and she gets third place. McCain gets the nomination after a tough fight with Romney or Guiliani or Pawlenty(the one of these coming in second). During the campaign, Whitman's help for Bush have lowered the collapse somewhat, causing as its major effect for state branches of Lehman Brothers to be remaining. Say, Whitman helps Bush by keeping Lehman Brothers running in the Industrial Midwest, where its jobs were important.

Thus, Bush's ~30% approval ratings are a contrast to Whitman's 40%/47% approval/disapproval ratings. McCain chooses Whitman, who then resigns from the Vice Presidency, giving Whitman ~46%/44% approval rating.

[Kingpoleon]

Who is Whitman? I was thinking Christine Todd but you said "he".

That was a typo.

Now, though, imagine a scenario where Clinton wins Iowa, Sanders wins N. H., Biden wins S. C., and O'Malley wins Nevada. It goes on, and on, and at the convention the balloting goes:

Clinton: 30.2%
Sanders: 27.4%
O'Malley: 22.1%
Biden: 19.1%

It goes on, and on, but eventually Booker is nominated as the compromise candidate on this ballot:

Booker: 50.5%
O'Malley: 23.2%
Biden: 20.1%
Other: 6.2%

Booker chooses Biden as his running-mate.