The Memo: Democratic field boils down to four-horse race

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 MoultonSeth MoultonThe Hill's Morning Report - Presented by Facebook - Markey defeats Kennedy; Trump lauds America's enforcers in Wisconsin Moulton fends off primary challenges in Massachusetts Portland: The Pentagon should step up or pipe down 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 BidenJoe 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, Sen. Elizabeth WarrenElizabeth 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.), Sen. Bernie SandersBernie 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.) and Sen. Kamala HarrisKamala 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.).

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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 BookerCory 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.) 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 ButtigiegPete 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), 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 SwalwellEric Michael SwalwellSwalwell 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 Swalwell: Barr has taken Michael Cohen's job as Trump's fixer MORE (D-Calif.), former Colorado Gov. John HickenlooperJohn HickenlooperCook Political Report shifts Colorado Senate race toward Democrat Willie Nelson playing at virtual fundraiser for Hickenlooper Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' MORE (D), Washington Gov. Jay InsleeJay 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 TrumpDonald 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, 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.