In his first extensive comments about a possible run for the Democratic presidential nomination, Biden said, "I can't look you straight in the eye now and say I know I can do it."

Biden, 72, has been huddling with advisers for weeks to determine whether he will challenge Democratic front-runner Hillary Clinton, who has seen her favorability ratings plummet over her use of a private email server while working as the nation's top diplomat.

"The most relevant factor in my decision is whether my family and I have the emotional energy to run," said Biden, who lost his son, Beau, to cancer earlier this year. "The factor is, can I do it?"

"The honest to God answer is I just don't know."

Biden was in Atlanta on Thursday night to deliver a foreign policy lecture.