In an excerpt from his first joint interview with running mate Kamala Harris, the Democratic nominee was asked by ABC’s David Muir about the president’s attacks on his mental agility. Mr Trump has called the former vice president “slipping” and “barely there.”
“Watch me, Mr President, watch me. Look at us both. Look at us both, what we say, what we do, what we control, what we know, what kind of shape we're in,” Mr Biden said.
“I think it's a legitimate question to ask anybody over 70 years old whether or not they're fit and whether they're ready. But I just, only thing I can say to the American people, it's a legitimate question to ask anybody. Watch me,” he added.
Throughout the primary campaign, Mr Biden referred to himself as a “transition candidate”, who would usher in a new generation of younger Democratic leaders. But when questioned about his long term plans, the 77-year-old also said he would “absolutely” be open to serving two terms in the White House should he win in November.
“We haven’t spent nearly enough time building the bench in the Democratic Party. In any state where we have competitive people running for the Senate and the House and the legislature, it's important the Democratic Party invest there,” he said.
“So [what] I want to do is make sure when this is over, we have a new Senate, we won back statehouses, we’re in a position where we transition to a period of bringing people up to the visibility that they need to get to be able to lead nationally. And that's about raising people up. And that's what I'm about.”
“So you're leaving open the possibility you'll serve eight years if elected?” Mr Muir added.
“Absolutely,” Mr Biden replied.
The interview with Mr Biden with Ms Harris will be broadcast on Sunday evening on ABC.