Biden to ABC's David Muir on Trump's attacks on mental fitness: 'Watch me, Mr. President'

Biden, Harris' first joint interviews air tonight at 8 p.m. ET on ABC

“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,” Biden told Muir during a joint interview with Harris in Wilmington, Delaware.

Biden would be 78 on Inauguration Day -- the oldest president ever elected if successful in November. Biden has called himself a transition candidate, but when Muir pressed him on what that meant, Biden said it did not mean a one-term president.

“We haven’t spent nearly enough time building the bench in the Democratic Party,” Biden said, recalling his commitment to campaign in states not consistently carried by Democrats, including Georgia, North Carolina and Florida.

“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,” Biden continued. “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?” Muir pressed.

“Absolutely,” Biden replied.

Watch “The Ticket: The First Interview,” a special edition of “20/20,” airing Sunday, Aug. 23 at 8 p.m. ET on ABC, in which “World News Tonight” anchor David Muir and “Good Morning America” co-anchor Robin Roberts conduct the first joint interviews with Democratic presidential nominee Joe Biden and his running mate Sen. Kamala Harris.