reporter: but the former vice president is not dwelling on the exchange tonight, saying his age is fair game. last night was the closest we came to a debate, okay? we actually had an open debate on health care. reporter: castro defending his words but not repeating them after being widely panned for a thinly veiled swipe at brydenid memory. reporter: after the debate, senator cor which booker also raising questions about biden s fitness for office. there are a lot of people concerned about joe biden s ability to carry the ball across the end line without fumbling. reporter: but reversing course today. forgive me if my football metaphor is being taken out of context but the reality is i want to get into the end zone. i think we need to win. reporter: the democratic debate in houston last night proved at least one thing. the 2020 primary fight is likely to go the distance driven by deep divisions about how far left the party is willing to go. front and center is health ca