The race for the Democratic presidential nomination is gathering pace.

The first debates will be held next month in Miami, even though the Iowa caucuses will not take place until February 2020.

The field grew to 24 last week when New York City Mayor Bill de Blasio Bill de BlasioOVERNIGHT ENERGY: California seeks to sell only electric cars by 2035 | EPA threatens to close New York City office after Trump threats to 'anarchist' cities | House energy package sparks criticism from left and right EPA threatens to close New York City office after Trump threats to 'anarchist' cities New Year's Eve in Times Square to be largely virtual amid pandemic MORE announced his candidacy.

Who are the top contenders?

1. Former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE

Biden has surprised skeptics with the extent of his early strength.

His rallies have been largely gaffe-free, his initial fundraising totals — $6.3 million in his first 24 hours — have been formidable and he has vaulted to a big polling lead.

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In four major recent polls — from Monmouth University, Quinnipiac University, Morning Consult and Fox News — Biden held a greater than two-to-one lead over his nearest challenger, Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.).

Biden’s appeal is simple: He bills himself as the best candidate to beat President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE, in part because of his roots in Scranton, Pa., and his cultural empathy for voters in the Rust Belt and Upper Midwest.

Trump has attacked Biden on Twitter as “Sleepy Joe.” But those attacks have boosted Biden’s cachet among Democrats, who see them as proof that the president fears him.

Biden’s appeal to nonwhite voters is, so far, stronger than anyone else in the field — in part, perhaps, a tribute to his loyal service to former President Obama.

Biden’s critics on the left insist his standing is falsely inflated right now and that he will fall to earth as his past record on everything from criminal justice to credit card companies becomes more widely known.

The 76-year-old former vice president has also had some verbal stumbles, as when he briefly referred to the late Margaret Thatcher, rather than Theresa May Theresa Mary MayAre US-Japan relations on the rocks? Trump insulted UK's May, called Germany's Merkel 'stupid' in calls: report Bolton says Boris Johnson is 'playing Trump like a fiddle' MORE, as the prime minister of the United Kingdom.

Overall, however, it’s clear Biden is the front-runner.

Previous ranking, Feb. 19: 2

2. Sen. Bernie Sanders (I-Vt.)

The Vermont Independent has held on to his position as the main standard-bearer of the left, despite facing increasing competition from Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass.).

Love him or hate him, Sanders is consistent. He is committed to the same democratic socialist ideals he has espoused for decades, he is as acerbic as ever about the media’s tendency to focus on personality over policies and many of his supporters display an intense devotion to him.

Sanders also has enormous resources. His campaign raised $18.2 million in the 41 days between his launch and the end of the first quarter.

But can he recapture the same kind of magic that powered his 2016 campaign against eventual nominee Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE?

The recent Monmouth University poll, conducted May 16–20, indicated support for him had declined by 5 points since the month before.

To win, Sanders needs the left to coalesce behind him. Warren’s rise suggests the opposite is happening.

Previous ranking: 3

3. Sen. Elizabeth Warren (D-Mass)

Warren is enjoying real momentum. In polls, she has broken out of the pack to become the clear No. 3.

Warren’s skill as a campaigner is evident in warmly received stump appearances.

Her campaign has also shown a deft touch for social media and popular culture. In a video posted on Twitter, she discussed “Game of Thrones” with progressive icon Rep. Alexandria Ocasio-Cortez Alexandria Ocasio-CortezWells Fargo CEO issues apology after saying there was a 'limited pool of Black talent' Brand responds to Trump claim protesters throw tuna cans at police: 'Eat em, don't throw em' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context MORE (D-N.Y.). In another tweet, she responded to comedian Ashley Nicole Black’s request for a plan to fix her love life.

Those flippant moments work because no-one doubts Warren’s intellect — she is a former Harvard Law School professor — or her policy chops.

Warren has rolled out policies on a broad range of issues, which she is able to explain with specificity and accessibility. Her campaign has underlined this strength, selling merchandise with the slogan, “Warren has a plan for that.”

Like Sanders, Warren is vulnerable to the suggestion that her left-leaning beliefs make her less electable than other front-line candidates. The memory of her shaky handling of the question of her Native American heritage has not disappeared.

For all that, though, Warren is snapping at Sanders’s heels. If she were to supplant him as the leading progressive candidate, the landscape of the race would shift.

Previous ranking: 5

4. Sen. Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (D-Calif.)

Harris topped The Hill’s rankings in February.

At the time, she had been boosted by a huge launch rally in Oakland, a sense that she brought more charisma to the race than any other candidate — and the fact that Biden had not yet declared.

Since then Harris’s fortunes have stagnated.

One persistent criticism is that she is too prone to caution. Her propensity to say she is willing to “have a conversation” about certain hot-button issues has been an inviting target for her skeptics.

Harris has also clarified or shifted her positions on several issues, such as whether her vision for “Medicare for All” involves the outright elimination of private insurance or whether police shootings should be investigated by independent prosecutors rather than local district attorneys.

Harris, the top-performing black candidate in the race, is so far receiving adequate but not outstanding support from nonwhite voters.

It’s far too early to count Harris out, but she needs some big moments that could push her closer to the very top of the pack.

Previous ranking: 1

5. South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegThe 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 Hillicon Valley: FBI, DHS warn that foreign hackers will likely spread disinformation around election results | Social media platforms put muscle into National Voter Registration Day | Trump to meet with Republican state officials on tech liability shield MORE (D)

A Buttigieg Boomlet has propelled the 37-year-old mayor from near-obscurity to the higher tiers of the field. Can he go any further?

Buttigieg has embarked on an exhaustive round of media appearances, which have showcased his chief assets: likability, an ease and clarity before the cameras, and a center-left appeal to comity and cooperation.

But there are real doubts about Buttigieg’s ability to appeal beyond the upscale whites who form the main pillar of his support. Some racially-tinged controversies during his tenure in South Bend could further hamper his appeal to non-white voters.

Previous ranking: N/A

6. Former Rep. Beto O’Rourke (D-Texas)

O’Rourke is already seeking to reboot his campaign, appearing at a CNN town hall and on ABC’s “The View” and expressing regret for telling Vanity Fair that he was “born" to run for the presidency.

The effort comes after the initial hype over O’Rourke fell flat.

The kind of retail campaigning that helped him in his Senate bid against Sen. Ted Cruz Rafael (Ted) Edward CruzCruz blocks amended resolution honoring Ginsburg over language about her dying wish Trump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP MORE (R-Texas) last fall has not broken through this time. Nor has O’Rourke enjoyed the same kind of viral moment as he did in the Senate race, when his defense of NFL players protesting racial injustice became a sensation.

O’Rourke could come back, but he is not an especially strong debater and he faces the possibility that Buttigieg — whose appeal is demographically similar — may have stolen his thunder.

Previous ranking: 4

7. Sen. Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.)

Booker has struggled to find a place for himself in the crowded primary field.

Having started his campaign talking about the transformative power of “radical love,” he has more recently sharpened his appeal — and his willingness to attack others.

He has taken a strong stance on some issues, including the protection of abortion rights.

But it’s not quite clear what the rationale for a Booker candidacy — or a Booker presidency — actually is.

Previous ranking: 6

8. Sen. Amy Klobuchar Amy KlobucharSocial media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Minn.)

Klobuchar’s bid is predicated on the idea that Democrats are willing to choose a centrist from the heartlands as the best candidate to take on Trump.

She is a lot more circumspect than her more progressive rivals when it comes to issues such as the Green New Deal and Medicare for All. She has referred to herself as “Heartland Amy” and she, like Buttigieg, has done a Fox News town hall.

Klobuchar’s appeal does make her distinctive to some degree. But Biden’s dominance among centrist voters seriously curtails Klobuchar’s room to run.

Previous ranking: 7

9. Former Secretary of Housing and Urban Development Julián Castro

Castro got off to a slow start in his campaign and usually polls in the low single digits. But that’s better than some better-known candidates, including Sen. Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (D-N.Y.), who has failed to register at all in several recent surveys.

Castro has made opposition to Trump’s immigration policies one of his signature issues. He has also sought to play up his connection to Obama, in whose Cabinet he served.

Will that take him far? Probably not. But a respectable performance in the field could put him on someone’s shortlist for vice president.

Previous ranking: N/A

10. Businessman Andrew Yang Andrew YangDoctor who allegedly assaulted Evelyn Yang arrested on federal charges The Hill's Campaign Report: Biden weighs in on police shootings | Who's moderating the debates | Trump trails in post-convention polls Buttigieg launches his own podcast MORE

Yang, a rank outsider, has at least nurtured his own small band of supporters — the self-proclaimed Yang Gang.

Yang has an idiosyncratic approach, for sure — boosting his profile by appearing on well-known podcasts rather then cable news, for example.

He has bemoaned the power of big tech companies. He has suggested a new value-added tax on corporations, with the proceeds dispensed as a universal basic income.

It is virtually inconceivable that Yang will be the nominee. But he has at least made some kind of imprint on a race where many others have barely been noticed.

Previous ranking: N/A

Other candidates: