MIAMI (Reuters) - For months, the 2020 Democratic campaign seemed mostly placid, even cordial. But tensions over race, gender, age and ideology simmered beneath the surface.

Former Vice President Joe Biden gestures as he speaks during the second night of the first Democratic presidential candidates debate in Miami, Florida, U.S. June 27, 2019. REUTERS/Mike Segar

At Thursday’s presidential debate, those frictions came to the fore – and Joe Biden bore the brunt.

The former vice president, 76, entered the debate as the front-runner, having led the pack of more than 20 Democratic candidates since he joined the race in April. But he has faced questions about whether he is too old, too white and too wedded to a bygone political age to be the nominee in the 2020 election for a party that increasingly prizes progressivism and diversity.

Biden’s opponents hammered away at those potential vulnerabilities, raising the prospect that the candidate who looked so formidable just weeks could be brought down to earth.

In the evening’s most memorable exchange, Kamala Harris, a black U.S. senator from California, blasted Biden for comments he had made about working with segregationists in the Senate and for opposing forced school busing almost 50 years ago.

Harris, 54, referenced herself as a little girl who was bused to school, highlighting in stark relief that she was a child when Biden was already a U.S. senator.

Biden tried to fight back, talking up his long record of supporting civil rights legislation. But he seemed defensive, rattled and wounded.

The dynamic persisted. Other than 77-year-old Senator Bernie Sanders, Biden shared the stage with candidates as many as four decades younger than him.

U.S. Representative Eric Swalwell, 38, who trails far behind Biden in opinion polls, first went after the senior statesman. “You can’t count on the people who have been in government for the last 30 years,” he said, calling on Biden to “pass the torch” to a new generation of Democrats.

Biden replied: “I’m still holding on to that torch.”

His remark drew laughter from the audience. But the exchange exposed how Biden has, in fact, refused to yield the spotlight to younger candidates.

EXPERIENCE AT ISSUE

Biden spent almost 40 years in the Senate and two terms as Barack Obama’s vice president. On Thursday, all that experience became a target his opponents were eager to exploit.

U.S. Senator Michael Bennet of Colorado blasted Biden over the 2013 spending deal Biden, as vice president, struck with Republicans, a deal he often touts on the campaign trail as an example of bipartisanship.

Bennet asserted the agreement favored Republicans by extending tax cuts and enacting spending caps. “It was a terrible deal for America,” he said.

Kirsten Gillibrand, a 52-year-old U.S. senator from New York, implied Biden had been wrong in his longtime support of the Hyde Amendment, which bars the use of federal funds for most abortions. Biden, under pressure from abortion-rights activists, recently changed his position on the law.

Harris also took issue with the record of Obama and Biden during their administration on deportations of immigrants in the country illegally, suggesting that Biden was not in sync with the current Democratic Party.

Since its beginning, Biden’s campaign has tried to stay above the fray, to portray his experience as an unbeatable asset and to paint the nomination as inevitable. Biden maintained his centrist record would position him best to defeat President Donald Trump next year.

Earlier in the day, his campaign insisted Biden was prepared to defend his record and train his focus on Trump. But his tense exchange with Harris appeared to disrupt that plan.

If there was ever an aura of inevitability about Biden, the debate on Thursday went a long way toward dispelling it.

The evening is likely to boost the prospects of Harris, whose campaign had seemed stalled. South Bend, Indiana, Mayor Pete Buttigieg, also turned in a consistently calm and measured performance.

Still, Biden remains a formidable contender. He is highly popular with a large swath of the older Democratic electorate and far ahead of the rest of the pack in national polls.

He faced intense questions from the media earlier this year about unwanted touching of women and most recently his remark about segregationists. Each time, it appeared his support went undamaged.

One poor evening will not sink him, but it remains to be seen if he will suffer in the polls as a consequence.

Biden has an immediate opportunity to do some damage control with black voters. He will be in Chicago on Friday to address the Rainbow PUSH Coalition, the group founded by the Rev. Jesse Jackson.

But he leaves Miami on the defensive. That is not a first for his campaign - and almost assuredly not the last.