The 2020 Democratic candidates for president are beginning the most important week of their campaigns with the front-runner looking more vulnerable than ever.

Former Vice President 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 has seen his poll ratings dip amid controversies over hot-button issues such as race and abortion. His responses have left many Democrats dissatisfied, sparking fresh questions about his judgment — and about the durability of his support.

The rest of the field is also unsettled.

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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 had been the clear No. 2 to Biden, has come under increasing pressure from progressive rival 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.), who has surpassed Sanders in some polls.

The other top-tier candidates — 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.) and South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE (D) — have enjoyed their moments in the sun, but now must combat the sense that their campaigns are plateauing.

That should ensure plenty of drama when the candidates clash for the first time on the debate stage this week. A total of 20 candidates — 10 on each night — will debate in Miami on Wednesday and Thursday.

The debates come against a backdrop of foreign policy volatility, with tensions between 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 and Iran unresolved and liable to increase further.

Trump, who officially launched his reelection campaign with a huge rally in Orlando, Fla., last week, can also be expected to intensify his Twitter fusillades against the Democrats running to oust him.

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The most recent major poll of Democratic voters, from The Economist and YouGov, showed Biden leading with 26 percent, followed by Warren with 16 percent, Sanders with 12 percent, Buttigieg with 9 percent and Harris with 7 percent.

But that poll was taken before Biden’s latest stumble, when he talked about his affable relationship with the late Sen. James Eastland (D-Miss.), a segregationist. Biden also praised another now-deceased Southern segregationist, Sen. Herman Talmadge (D-Ga.).

The remarks sparked uproar, including a request from 2020 candidate Sen. Cory Booker Cory Anthony Booker3 reasons why Biden is misreading the politics of court packing Bipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility MORE (D-N.J.) for Biden to apologize. Biden has refused to do so.

The misstep, coming so soon after another furor when the former vice president made a U-turn on his previous support for the anti-abortion Hyde Amendment, has left a lot of Democrats skeptical that he will remain the undisputed leader of the pack for long.

“We are now seeing a lot of doubt about Biden and Biden’s campaign,” said Democratic strategist Joel Payne. “His campaign has raised red flags with many Democrats.”

Another party strategist, who requested anonymity to speak candidly, put it even more strongly.

“Biden’s candidacy right now is on quicksand and his campaign is making it worse,” this source said, before going on to compare Biden to another Democratic front-runner who failed to make it to the White House.

“They are running the Hillary Redux campaign. What I mean by that is, they’re playing it safe, there are unforced errors and there is — clearly, I would say — a lack of discipline and aggressiveness.”

One key question for the debates is how Biden will cope with any pointed jabs that come his way. He will be the focal point of the debate on the second night, where he will be jousting with Sanders, Buttigieg and Harris, among others.

“I think Biden’s challenge will be to just stay firm yet not seem overly confident,” said Karen Finney, who was a senior adviser to Democratic nominee 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 in 2016 but is not affiliated with any presidential candidate this cycle.

“I do think it is important that when you’re in the lead you show you are willing to work for it and aren’t taking anything for granted,” she said.

But whether or not Biden stumbles again, the battle to become his principal rival is already fierce.

Warren is clearly the candidate with momentum. Her strength on the stump and her abundance of policy plans — most recently for a ban on private prisons — has seen her move past an unsteady start to her campaign, when she was dogged by controversies over her past claims to Native American heritage.

Warren’s support is generally seen as coming at the expense of Sanders, who has not shown much sign of being able to grow his support from its initial levels.

“Sanders right now is bleeding support and he’s not losing that support to Biden,” said the Democratic strategist who wished to remain anonymous.

A strong debate performance from Sanders could change that at a stroke.

Warren, meanwhile, is the only one of the five top-polling candidates who is debating on the first night on Miami. Whether that helps her because she will be center-stage, or hurts her because she does not get the chance to land punches on her main rivals, remains to be seen.

For Buttigieg and Harris, who both debate on the second night, the need for a viral moment or at least some impactful exchanges is strong. Both candidates have a charismatic television presence and Harris, a former prosecutor, is a formidable debater.

In Harris’s case, however, a strong campaign launch has only translated into her being fourth or fifth in the field — a credible performance when there are 24 candidates running, but not one that makes her an instantly plausible winner of the nomination.

Buttigieg has followed a similar trajectory.

“Kamala Harris feels like the Marco Rubio Marco Antonio RubioGOP lawmakers distance themselves from Trump comments on transfer of power McConnell pushes back on Trump: 'There will be an orderly transition' Graham vows GOP will accept election results after Trump comments MORE of this cycle right now,” said a second Democratic strategist who would only speak anonymously. “She just hasn’t lost support, but it has been stagnant. She needs a moment here.”

The bar is different for other candidates outside the top five.

Figures such as Booker and former Rep. Beto O’Rourke (D-Texas) have failed to gain traction and will be desperate to change that.

Other candidates even further back in the field will be looking to gain some attention — and at least do enough to qualify for the next officially sanctioned debate, which is scheduled for next month.

An opinion poll last week from communications firm Park Street Strategies found 67 percent of likely Democratic voters believing there are too many candidates in the race.

The firm’s founder, Democratic strategist Chris Kofinis, told The Hill that this week’s debates are “going to force these candidates to either stand up or basically disappear. For a lack of a better way of putting it, Democratic voters are suffering from ‘TMC’ — too many candidates.”

The winnowing — and the real drama at the top of the field — is about to begin.

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