Mr Obama has largely stayed silent since leaving office in 2017, opting not to endorse anyone in the Democratic primary and only occasionally weighing in to criticise Mr Trump. "You can't do it when it's convenient and then dodge it when it's not".
Sen. Michael Bennet of Colorado saved most of his criticism for Trump, but said he anxious, too, that Trump would destroy Democrats if Harris's plan became the Democratic offer.
"Barack Obama knew exactly who I was", he said. Kamala Harris (D-Calif.), and a rematch between the two was anticipated after Harris slammed Biden for his record on busing in the most memorable moment of June's debate.
In one testy exchange, Biden shot back at Booker for his record as former mayor of Newark, saying Booker allowed the "stop and frisk" policing approach and hired someone associated with Rudy Giuliani, former New York City mayor and now Trump's lawyer.
U.S. Sen Kirsten Gillibrand came after Biden on women's issues at the Detroit debate Wednesday.
Other candidates also attacked Biden, some at the behest of CNN's moderators.
The ideological divisions gripping the Democratic Party intensified as presidential candidates battled over health care, immigration and race.
She faced criticism from all sides this week after releasing a competing plan that envisions a role for private insurance with strict government rules, but she wants to transition to a single-payer government-backed system within 10 years.
Biden met with patrons, MI lawmakers and black community leaders at Detroit One Coney Island the morning after fielding attacks form Democratic rivals in the second primary debate at The Fox Theatre.
"The only reason this particular part of the law is being abused is because of Donald Trump", Biden said.
Former Vice President Joe Biden listens as Sen.
Laughable as it may seem, Biden is the not-crazy one in the Democrat field.
The former vice president and DE senator said that he helped pass a bill that allocated $750 million for the Northern Triangle countries to try to alleviate the problems there so fewer people would migrate to the United States.
"I think the black community is no different from any other community in the sense they are being hurt badly", Biden said.
Democratic presidential candidate Julian Castro, left, speaks as Sen. Cory Booker of New Jersey, who sparred with Biden on criminal justice, particularly for a crime bill Booker said wrongly hit African Americans harder by punishing crack possession more than cocaine.
Castro, the former mayor of San Antonio, retorted, "It looks like one of us has learned from the lessons of the past and one of us hasn't".