MIAMI — The front-runner stumbled badly on Thursday.

In his first debate of the election season, former Vice President Joe Biden failed to cement his place as the favorite to win the 2020 Democratic presidential nomination — giving a wider opening to a small pack of legitimate rivals.

And over the span of two days, Sen. Kamala Harris, D-Calif., who dueled with Biden over his record on school desegregation, and Sen. Elizabeth Warren, D-Mass., who was widely seen as the top debater in Wednesday's first heat of the two-day event, created opportunities for themselves.

Heading into debate week, Biden stood in first place in the Real Clear Politics average of national polls at 32 percent, with Vermont Sen. Bernie Sanders at 16.9 percent, Warren at 12.8 percent and Harris at 7 percent.

It was Harris who rattled Biden the most thoroughly, pressing him in a testy exchange over his record on school desegregation. As the only black candidate on the stage, Harris recounted her own experience with integration and told Biden it was "hurtful" that he had worked with segregationist senators and led anti-busing efforts.

"She was very effective in that moment," said one of the other candidates, Sen. Michael Bennet, D-Colo.

Biden accused her of a "mischaracterization" of his record, but reiterated the argument he made at the time: that the federal government should not have been empowered to force schools to use busing as a means of desegregation. Though it didn't come up, Biden pushed similar legislation designed to protect local schools from a federal requirement that they allow black children into the same classrooms as white children.

By the end of the back and forth, the usually loquacious Biden cut himself off.

"My time is up," he said abruptly. "I'm sorry."

But that wasn't the only time he found himself on the defensive.

The 76-year-old former senator absorbed shots from Rep. Eric Swalwell, D-Calif., about how long he's been on the national stage and, in more subtle fashion, from 77-year-old Sanders about his fortitude.

"The issue, if I may say, is not generational,” Sanders said. “The issue is who has the guts to take on Wall Street, to take on the fossil fuel industry, to take on the big money interests.”

Biden's aides and allies said they felt good about his performance but stopped short of claiming a major victory — a sure sign they knew he'd had rough patches.

"He had an extraordinary night ... now, look, as a front-runner going into a debate with nine people coming after you, nobody thought it would be easy," said Rep. Cedric Richmond, D-La., a co-chairman of Biden's campaign. "But I thought he did one remarkable job standing up."

Richmond, a former chairman of the Congressional Black Caucus, defended Biden's record on the issue that gave him his toughest moment on stage.

His position on busing, Richmond said, "was always clear" — "that African-Americans deserved and were entitled to a quality public education ... We needed to stop red-lining so black people could move into white communities and better school districts."

In the midst of her back-and-forth with Biden, Harris personalized the issue, creating an indelible political moment for both candidates. She spoke of a little girl who was part of a wave of students integrating schools.

"That little girl was me," she said.

At once, the line was a dagger for him and exactly the kind of touchstone she needed to let voters in to her personal narrative.

There are three big questions now for Biden: whether his poll numbers will suffer at all for a lackluster debate performance, whether he has the wherewithal to rebound and whether he has given Harris, Warren or any other candidates the opening they need to cannibalize his base.

This is his third presidential campaign — he ran in 1988 and 2008 — and he knows as well as anyone that the primary season is a long one. The first contest on the primary calendar, the Iowa caucuses, won't happen for seven months.

But because his main selling point among Democrats is the idea that he's the most electable candidate, he can ill afford to show vulnerabilities. His rivals exposed several of them Thursday.

He can expect more of the same in the coming debates.