According to conversations with a dozen Democrats — a mix of elected officials, formal campaign advisers, and outside strategists, most of whom were granted anonymity to share their candid assessments — Biden pines for a partner who could be to him what he was to Obama: a friend and confidante who subsumed his political interests to those of his boss.

The elusive hunt for Biden’s Biden has recently pushed Susan Rice into the top tier of candidates. As Obama’s national security adviser for his entire second term, Rice and Biden worked closely together on an almost daily basis, making her the only potential running mate who Biden knows so intimately.

But very few of the other candidates come close to meeting that test, which means that the one-on-one candidate interviews could be unusually important. A dark horse candidate who aces her oral exam with the nominee could end up as the finalist — and considering Biden’s commanding status in the race, as well as his age — the most likely person in America to be the 47th president.