Joe Biden wonders if he has 'emotional energy' to run for president

U.S. Vice President Joe Biden said on Thursday he is trying to determine whether he has the emotional energy to mount a White House bid in 2016 but cannot say yet whether he can do it.

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."

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Biden, 72, has been huddling with advisers for weeks to determine whether he will challenge Democratic front-runner Hillary Clinton, who has seen her favourability 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 after concluding a two-day visit to the political battleground state of Florida, where he avoided any discussion of a possible White House run.

Biden said he would not be swayed by questions about whether he could raise enough money or mount an effective organization after getting a late start. The only factor, he said, was his and his family's commitment.

"Can my family undertake what is an arduous commitment?" he said.

"Unless I can go to my party and the American people and say I am able to devote my whole heart and soul to this endeavour, it would not be appropriate."

Biden, who served as a Senator for Delaware for over 35 years, ran for the Democratic nomination in 1988 and 2008, although in each case his bid ended relatively early in the process.

He has stepped up his public schedule, and will travel to Pittsburgh on Monday for Labour Day celebrations and New York later in the week to appear on Stephen Colbert's talk show.

But he said he did not know when he would make a decision.

"There is no way to put a timetable on it," he said.