The quest for the Democratic presidential nomination is becoming a four-horse race.

Marginal candidates at the bottom of the large field have begun to drop out — Rep. Seth Moulton Seth MoultonOvernight Defense: Nearly 500 former national security officials formally back Biden | 40 groups call on House panel to investigate Pentagon's use of coronavirus funds The Hill's Morning Report - Presented by Facebook - Markey defeats Kennedy; Trump lauds America's enforcers in Wisconsin Moulton fends off primary challenges in Massachusetts MORE (D-Mass.) became the latest to do so on Friday — and a deadline is looming this week to qualify for the third round of debates in Houston in September.

Meanwhile, at the head of the pack, only a major surprise would deliver the nomination to anyone outside the top quartet: former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.), Sen. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.) and Sen. Kamala Harris Kamala HarrisTexas Democratic official urges Biden to visit state: 'I thought he had his own plane' The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements A game theorist's advice to President Trump on filling the Supreme Court seat MORE (D-Calif.).

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Those four lead the polls and suck up media attention. They also offer Democratic voters a potential standard-bearer from the left (Warren or Sanders), the center (Biden) or somewhere in between (Harris). And, of course, either Warren or Harris could become the first female president.

Put it all together and it is difficult to see an opening for other contenders to make a serious run at the nomination.

Candidates who launched with high hopes, notably Sen. Cory Booker Cory Anthony BookerThe movement to reform animal agriculture has reached a tipping point Watchdog confirms State Dept. canceled award for journalist who criticized Trump 3 reasons why Biden is misreading the politics of court packing MORE (D-N.J.) and former Rep. Beto O’Rourke (D-Texas), have put in months of campaign-trail toil without seriously threatening to crack the top tier.

The one surprise has been the strength of 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), who bested everyone in fundraising during the second quarter.

But Buttigieg has not developed the kind of momentum that suggests he could go all the way. He has shown little evidence he can expand his support beyond upscale whites, and concerns persist that voters would see him as just too young, at 37, to be commander in chief.

In addition to Moulton, Rep. Eric Swalwell Eric Michael SwalwellHouse to vote on resolution affirming peaceful transition of power Swalwell calls for creation of presidential crimes commission to investigate Trump when he leaves office 'This already exists': Democrats seize on potential Trump executive order on preexisting conditions MORE (D-Calif.), former Colorado Gov. John Hickenlooper John HickenlooperThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Cook Political Report shifts Colorado Senate race toward Democrat Willie Nelson playing at virtual fundraiser for Hickenlooper MORE (D), Washington Gov. Jay Inslee Jay Robert InsleeBarr asked prosecutors to explore charging Seattle mayor over protest zone: report Bottom line Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE (D) and former Alaska Sen. Mike Gravel (D) have all dropped out well in advance of the first contests. The Iowa caucuses are scheduled for Feb. 3.

Moulton sees the field of real contenders as comprising just three people.

“I think it’s evident that this is now a three-way race between Biden, Warren and Sanders, and really it’s about how far left the party should go,” he told The New York Times as he announced his withdrawal.

Biden retains a sizable lead in national polls, but the big question is whether that support will endure as Democrats focus more sharply on the campaign in the months between now and the Iowa caucuses.

In two televised debates so far, Biden performed weakly in the first and adequately in the second.

The former vice president has a lifelong propensity for gaffes, but missteps now spark questions about his age and his robustness to compete against Trump.

Most recently, Biden, 76, referred to the assassinations of Martin Luther King Jr. and Robert Kennedy as having taken place in the 1970s. King and Kennedy were killed in 1968. On Saturday, Biden mistakenly referred to the town in which he was campaigning — Keene, N.H. — as being in Vermont.

Democratic insiders also note that Biden will face new scrutiny once all his main challengers are onstage with him in the debates. Biden has yet to appear on the same stage as Warren, seen by many Democrats as his strongest challenger.

Mark Longabaugh, a Democratic strategist who has worked for several past presidential campaigns but is not affiliated with any candidate this cycle, said that future debates “are going to be very critical for Biden.”

But he added: “It’s also going to be about the white-hot heat of the process in December, January, February. It is going to be the enormous intensity of that, and the focus that Biden is going to be under. Every comment is going to be scrutinized.”

Other Democrats share Longabaugh’s skepticism.

Biden “would be the most flawed Democrat to win a contested primary like this in a long time,” said strategist Joel Payne.

But Biden and his allies believe his argument that he is the best candidate to take on Trump will win the day.

His first TV ad in Iowa asserts that “we have to beat Donald Trump, and all the polls agree Joe Biden is the strongest Democrat to do the job.”

His competitors argue that electability alone is not enough.

"Democrats shouldn't modulate our values or try to triangulate our policy positions based upon what some pundits say is electability,” Booker told CNN last week.

The most recent major national poll from The Economist and YouGov showed an extremely close contest, with Biden at 22 percent support, only just shading Sanders (19 percent) and Warren (18 percent).

But, underlining the uncertainty that still hangs over the contest, a CNN-SSRS poll just days earlier found Biden 14 points clear of the same two candidates, with Biden at 29 percent, followed by Sanders's 15 percent and Warren's 14 percent.

In any event, the battle between Sanders and Warren — each one fighting to be the standard-bearer of the left — will be vital to the race’s outcome. There is a distinct possibility that they might split the progressive vote, which would ease the path for Biden, the clear choice among more centrist Democrats.

Warren has appeared to gain momentum recently and has surpassed Sanders in several polls. But Sanders retains a committed base of support, and his allies insist his chances are being underestimated.

Harris, meanwhile, must somehow thread a path through the three leading candidates. The California senator's best chance may come if Biden stumbles badly. Such a scenario would sharpen her appeal to both the center-left and more liberal factions of the party.

Still, her boost in the polls after the first debate has faded, and questions about her core beliefs, on everything from criminal justice to health care, persist.

The fight will only get fiercer from here.

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