MIAMI — The 2020 Democratic presidential race has burst wide open after two debates here this week.

Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE, the former vice president who has until now been the unarguable front-runner, has been badly weakened. Sen. Kamala Harris Kamala HarrisHundreds of lawyers from nation's oldest African American sorority join effort to fight voter suppression Biden picks up endorsement from progressive climate group 350 Action 3 reasons why Biden is misreading the politics of court packing MORE (D-Calif.) has reinvigorated a campaign that was at risk of becoming stagnant. And Sen. Elizabeth Warren Elizabeth WarrenHillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Democratic senators ask inspector general to investigate IRS use of location tracking service MORE (D-Mass.) accelerated her momentum with a dominant performance in the first and less contentious debate on Wednesday.

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The eventual nominee to take on President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE is likely to be one of those three people or Sen. Bernie Sanders Bernie SandersThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Bernie Sanders: 'This is an election between Donald Trump and democracy' The Hill's 12:30 Report: Trump stokes fears over November election outcome MORE (I-Vt.), who acquitted himself solidly but unspectacularly on Thursday.

Other candidates had impressive showings here — none more so than former Housing and Urban Development Secretary Julián Castro on Wednesday — but the gap between the big figures in the field and the trailing pack seems wider than ever.

There is no question that Biden was the major loser.

His halting performance on Thursday shows him to be newly vulnerable — and resurrects the ghosts of previous candidates who have been strong favorites in the early stages of primary fights yet failed to capture the nomination.

During the 2008 cycle, then-Sen. Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE (D-N.Y.) led then-Sen. Barack Obama Barack Hussein ObamaObama warns of a 'decade of unfair, partisan gerrymandering' in call to look at down-ballot races Quinnipiac polls show Trump leading Biden in Texas, deadlocked race in Ohio Poll: Trump opens up 6-point lead over Biden in Iowa MORE (D-Ill.) by wide margins for much of 2007 — only to be pushed into third place in the Iowa caucuses as Obama stormed home. It was a shock from which her campaign never fully recovered.

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Notably, Clinton's slide may have begun with a debate misstep in October 2007, when she gave an evasive answer on the question of whether illegal immigrants should be entitled to driver's licenses.

On the other side of the aisle, former Florida Gov. Jeb Bush (R) took an early lead four years ago — only to be swiftly overtaken by Trump.

Despite lavish spending from a super PAC that burned through almost $100 million on his behalf, Bush withdrew after the third contest, the South Carolina primary, never having placed better than fourth.

Biden can still avoid such dismal fates, of course. But he faces several interlocking problems.

One is that the core rationale for his candidacy is that he is the strongest candidate to beat Trump. If he is now judged to be an unsteady debater, that argument may be holed below the water line.

Biden could recover with better performances in the future. He could lean even more heavily into his supposed empathy for voters in the electorally vital Rust Best and Upper Midwest.

But Biden, seeking to show a front-runner’s strength, was revealed as weak in his first big test. The fallout could be severe.

Another difficulty for Biden is that his worst moment came on the most sensitive topic in American public life: race.

Harris excoriated him for his warm words for two now-deceased Southern segregationists, former Sens. James Eastland (D-Miss.) and Herman Talmadge (D-Ga), as well as for his opposition to school busing.

Biden’s defense — that Harris was mischaracterizing his record and that his opposition to busing was based on a belief that local authorities, not the federal government, should decide the policy — was ineffective.

She also invested the exchange with real emotional power by calling his words “hurtful” and noting that she herself had been a beneficiary of busing as a child.

By Friday, Biden was seeking to ameliorate the damage, appearing at Rev. Jesse Jackson’s Rainbow PUSH Coalition in Chicago, invoking his loyal service to former President Obama and asserting that “the discussion in this race today shouldn’t be about the past.”

That might be a stronger argument if the 76-year-old Biden didn’t invoke the more favorable parts of his own history so frequently.

Jackson, for his part, praised Harris and described Biden as “on the wrong side of history” on busing in a CNN interview.

As if all that were not enough, Biden’s faltering performance was seen by a huge number of people.

The audience for the second debate across NBC News, MSNBC and Telemundo totaled 18.1 million people, the highest-ever figure for a Democratic primary debate. Wednesday’s event drew around 15.3 million viewers.

The big audiences should help Harris as much as they hurt Biden.

In addition to the viral exchange with the former vice president, Harris commanded the stage, reprimanding her rivals for squabbling with each other and speaking in vivid terms about the difficulties American families face because of the cost of health care.

As a candidate, "you want to have a breakout moment. Sen. Harris absolutely did," said Karen Finney, a senior adviser to Clinton during the 2016 campaign who is not affiliated with any candidate in this cycle.

Harris's campaign spent Friday emailing reporters roundups of the praise she had received. It also sent out a fundraising appeal that had “Momentum” as its subject line and described Harris as “on fire” at the debate.

A Harris campaign aide had previously told CNN that the campaign raised more money on Thursday, after the debate, than any day of the campaign other than Harris's launch and the day after it.

In concrete terms, a rise in black support for Harris, especially at Biden’s expense, could fundamentally reshape the race. The California senator has not so far drawn noticeably large African American support despite being the most prominent black candidate.

Biden’s appeal, especially to older black voters, has been a roadblock of sorts for Harris. That roadblock may be lifting.

Biden’s damage-limitation exercises went beyond his appearance at Jackson’s organization. In an email to supporters Friday afternoon, he said he had “heard” Harris and that he respected her.

“For my entire career, I’ve fought my heart out to ensure that civil rights, voting rights, and equal rights are enforced everywhere,” Biden wrote.

But now that he is politically injured, his rivals are circling.

How well, and for how long, can he hold them off?

The Memo is a reported column by Niall Stanage, primarily focused on Donald Trump’s presidency.