Washington (CNN)Joe Biden knew it before, but it's unquestionably true now: The burden is on him to prove he's the front-runner in the Democratic presidential race.
"He knows he has to do better," a Democratic ally who works closely with the Biden campaign said.
After waking up Friday to brutal headlines and tough critiques about his performance in the first debate, the former vice president realizes he didn't quiet the rising whispers among many Democrats who wonder if he's up to the task of surviving a bruising primary and taking on President Donald Trump, this ally said.
While Biden and his advisers point to several moments they believe were strong in the debate on Thursday evening -- like batting away the generational critique from Rep. Eric Swalwell with a smile -- they were still assessing the political fallout over the tense exchange with Sen. Kamala Harris on race.
Harris targeted Biden for invoking his ability to work with segregationist senators as an example of a more civil time in the US Senate. But her sharpest and most personal attack was over his past opposition to mandatory busing to desegregate schools.
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"There was a little girl in California who was part of the second class to integrate her public schools and she was bused to school every day," Harris said. "That little girl was me. So, I will tell you that on this subject, it cannot be an intellectual debate among Democrats. We have to take it seriously. We have to act swiftly."
Most alarming and puzzling to some Democratic admirers of Biden, including veterans of the Obama administration, was his embrace of state's rights on the question of school busing and the role of the federal government.
"You would have been able to go to school the same way because it was a local decision made by your city council," Biden told Harris.
"I did not oppose busing in America, what I opposed is busing ordered by the Department of Education," Biden added.
He sought to clean up that answer on Friday at a previously scheduled appearance in Chicago at a meeting of Jesse Jackson's Rainbow PUSH Coalition.
"I heard and I listened to and I respect Sen. Harris," Biden said at the event. "But, you know, we all know that 30 seconds to 60 seconds in a campaign debate exchange can't do justice to a lifetime committed to civil rights."
Biden said he never opposed "voluntary busing" and backed away from his debate-stage argument in favor of states' rights, saying that "I fought my heart out to make sure that civil rights, voting rights, equal rights are enforced everywhere" and that it's the "federal government's duty" to ensure their enforcement.
Jackson told CNN's Poppy Harlow on Friday before Biden's speech that Harris "was on point" in her criticism of the former vice president last night and said he intends to ask Biden about his past positions when they meet today in Chicago.
"I would think that Joe Biden must now address this in a real sense," Jackson said.
A senior Democrat who is close to Biden -- and fond of him -- responded with a two-word answer when asked about his debate performance.
"Not great," the Democrat said, speaking on condition of anonymity to avoid offending the former vice president. "The question is whether it was an off-night or whether he's just not sharp anymore."
That question, of course, can't be answered from one debate alone, but rather the collective view of his performance on the campaign trail and in other appearances.
Going forward, particularly in the month leading up to the CNN debate in late July, aides say Biden will step up his public visibility and schedule in his quest to prove he is deserving of the front-runner's mantle and to fire him up.
He's been spending most of his time raising money -- a critical necessity -- but his schedule is going to change after this weekend to far more campaigning, surrounded by voters in early states, aides say.
Democrats say one thing Biden can't change -- his age -- also remains among his biggest challenges.
"The more damaging thing for Biden was the age thing," said a senior Democrat who worked with Biden for years. "He took too long to reply and too many of his answers were around bills he introduced decades ago."
Still, the Democrat sympathetic to Biden said: "He is bruised, but far from out."
