(Bloomberg) -- Joe Biden remains the man to beat for the Democratic presidential nomination, but the party’s first debate exposed vulnerabilities that will test the former vice president’s front-runner status as the ground shifts beneath him.Biden’s rivals on the Miami stage took aim at his age, his 36-year voting record in the Senate, and nostalgic remarks about having civil relationships with segregationist senators. The 76-year-old elder statesman benefits from a deep reservoir of good will among Democrats, but the two nights of debate featuring an unprecedentedly diverse field also symbolized how rapidly his party is changing.Senator Bernie Sanders knocked Biden’s vote for the Iraq war. Representative Eric Swalwell, 38, told him it’s time to make way for a new generation. Senator Elizabeth Warren implicitly contrasted her vision of feisty liberalism with his brand of institutionalism. Senator Kamala Harris delivered a stinging attack over his past skepticism of busing to racially integrate schools.Biden on Friday played down the attacks by Harris.“I heard and listened to and I respect Senator Harris. But you know, we all know that 30 seconds to 60 seconds on a campaign debate exchange can’t do justice to a lifetime committed to civil rights,” he said at an event in Chicago with civil rights activist Jesse Jackson. “I’ve fought my heart out to ensure that civil rights and voting rights, equal rights are enforced everywhere.”“And that’s always been my position,” he said.Still, the debate on Thursday showed that Biden has a tall task ahead to preserve his strong support seven months before the 2020 contest begins in Iowa.His critics say his familiarity to voters, from eight years as Barack Obama’s vice president, gives a false sense of his true strength in the contest. These critics say that so early in the nominating process, voters are going to back Biden mainly because they know him best but that his support isn’t fervent, enduring or deep. “Kamala Harris showed Joe Biden is behind the times and not the Democratic Party’s most electable candidate by a long shot,” said Adam Green, co-founder of the Progressive Change Campaign Committee, an activist group that is backing Warren.That means Biden’s challenge will be to prove his support is strong enough to withstand the the rocky first debate. It remains an open question whether he can achieve that or whether his debate outing has shaken his supporters’ confidence that he’s the one to win the loyalty of Democrats and beat President Donald Trump.One key metric to watch is how he fares with donors. On Friday, a major San Francisco-based contributor, Tom McInerney, told CNBC he had notified the Biden campaign that he was pulling back his support after Biden’s comments about working with the segregationist senators and his history of opposing busing came to light.Biden’s task is all the more complicated because, while he has hardly changed, his party has — ideologically warming to more progressive and populist ideas, and shedding white working class voters while attracting more women, young people, blacks and Hispanics.The visuals at the debate told the story.Biden shared the stage with three women, two rivals in their 30s, one openly gay contender, and people of black and Asian ancestry. The large Democratic field, the most diverse ever, also includes potentially the first Latino and first Hindu president.Biden must persuade the party’s diverse base he can speak for them -- making his verbal misstep on the busing question all the more perilous because it suggested he could not relate to Harris’ position as the African-American girl who benefited from integration and busing.In Washington on Friday, Harris told reporters that the debate “covered a lot of issues and I’m looking forward to the next round,” which will be held on July 30 and 31 in Detroit.On Biden, she added: “He said what he felt.”The moment was a much-needed jolt for Harris, who has slipped to fourth or fifth place in recent Democratic surveys after a strong campaign launch in January that was widely believed to have the makings of a front-running bid for the White House.At a fundraiser last week in New York, Biden wistfully mentioned having a good working relationship with Democratic segregationists like James O. Eastland, even though they “didn’t agree on much of anything,” and cited such associations as examples of a time of civility when “the political system worked.”A central element of his pitch, repeated Thursday night, is building consensus between opposing factions and restoring “the soul of this nation” after “this president has ripped it out.”\--With assistance from Daniel Flatley.To contact the reporter on this story: Sahil Kapur in Miami at skapur39@bloomberg.netTo contact the editors responsible for this story: Joe Sobczyk at jsobczyk@bloomberg.net, Max Berley, John HarneyFor more articles like this, please visit us at bloomberg.com©2019 Bloomberg L.P.