“People were raising serious questions about his ability to be very aggressive against Trump,” said one person who spoke with Biden.“He’s listening. He seems to be more sure of himself after that first fiasco. I think he realizes now that he screwed up.”…

Part of the nervousness is rooted in the flatness of his first debate. Biden understood the stakes, was aware that he’d be a target and had the assistance of a top-notch team of debate-preppers. He even had Michael Sheehan, the best-known speech coach in the party, in his corner.

Still, Biden walked straight into Kamala Harris’ ambush, flailed awkwardly as he attempted to defend his record on busing and civil rights and looked anything but a front-runner.

It served as a vivid reminder of the former vice president’s headstrong ways and bad habits, acquired over decades in office. Sources who have worked to prepare him for debates in past years say he has a propensity for foregoing the advice of professionals and going off-script. It all creates an aura of uncertainty around Biden, a problem for a contender whose candidacy is built around the premise that he is the safe choice among a sprawling field of Democrats.