The explanations for the discrepancy run the gamut. The white Iowa and New Hampshire electorates play against Biden’s strength among ideological moderates and African Americans, some defenders argue. Skeptics say it shows that the voters watching him most closely are underwhelmed.

There's the fact that Biden pulled back on early state ad spending — both on TV and digitally — while competitors ramped up. Finally, the Trump factor: The president's reelection campaign has been running anti-Biden ads on TV in Iowa and more broadly over social media platforms like Facebook and YouTube.

Nationally, the Real Clear Politics average shows Biden up by 7.7 percentage points; in at least half of recent polls, he's leading by double digits. In Iowa and New Hampshire, however, it’s Warren who’s up — by 4.3 percentage points and 4 percentage points, respectively.

Some Democratic analysts think the dual tracks are unsustainable — and that the reckoning won't be pretty for Biden.

“If Biden tanks in Iowa and New Hampshire, the national polls will follow,” said Patrick Murray, who directs the Monmouth University poll. “The [national] numbers are ephemeral. They’re not anchored like the early state polls, which determine who breaks out on the national scene.”

But the states that immediately follow New Hampshire — South Carolina and Nevada— tell a different story. Biden has consistently dominated in South Carolina and steadily leads in Nevada. Biden advisers argue that’s because of more diverse electorates in those states: About 21 percent of Nevada’s caucusgoers are expected to be nonwhite, as are two-thirds of South Carolina’s primary voters.

"Trump's unprecedented attacks definitely don't help, but Biden resonates with diverse populations," said Amanda Loveday, the former director of the South Carolina Democratic Party who is now involved in a new pro-Biden super PAC. "Out of the 10 states with the highest population of nonwhite residents, Biden is leading in seven of them."

Here's a closer look at the factors driving Biden's polling disparity, one of the key dynamics of the primary so far:

When Trump attacks

In October, Trump started hitting Biden in early states with negative TV ads over his son Hunter’s work in Ukraine while the elder Biden was vice president. To date, the Trump campaign has spent $287,000 on TV in Iowa alone, according to Advertising Analytics; it has spent far more on digital ads over Facebook and YouTube.

How much of that went toward attacking Biden isn't clear, but it appears to be significant. Trump's campaign and the Make America Great Again Committee — a joint fundraising committee of the campaign and Republican National Committee — have spent more than $25 million combined since January. Digital ads targeting Biden on Ukraine have run on Facebook, YouTube and other platforms since October.

“We have to confront the awful reality now that Trump’s illicit campaign to use Ukraine to bloody the leading Democratic candidate may be in fact working,” said Simon Rosenberg, founder of the New Democrat Network and a senior strategist for the Democratic Congressional Campaign Committee in 2018.

A new super PAC backing Biden released a memo over the weekend echoing that sentiment, saying Trump “wants to choose the nominee, and we aren’t willing to let that happen.”

Advertising imbalance

Biden in August and September scaled down on digital advertising nationally and in early states, allowing four of his primary challengers to outspend him on digital ads. That includes Buttigieg, who spent $7.7 million — nearly twice as much as Biden — on digital ads from April to the first week of November. The same is true on the airwaves: As Trump was attacking him in Iowa in October, Biden spent just $55,000 on TV ads there that month, compared to more than $1 million each for Sanders and Buttigieg.

At that same time, Buttigieg leapfrogged Biden in polls. Biden supporters say recent Iowa poll numbers are not a sign of what’s to come but rather a reflection of a close race. While recent polls show Biden in fourth place, they also show him and two others — Buttigieg and Sanders — within the margin of error for second place.

John Anzalone, Biden’s chief pollster, disputes that Biden has dropped to fourth in Iowa; he thinks the former vice president is trailing only Warren in the state.

“Historically, if you look at 2004, 2008 or 2016, the early states are always more competitive because that’s where all the paid communications are,” Anzalone said. “Iowa is just naturally more competitive. There has never been a time when Iowa hasn’t bunched up at this time.”

Still, there’s no question Biden has lost ground in Iowa, steadily declining in local polling since the start of 2019.

Polk County Democratic Chairman Sean Bagniewski gave a different reason for Biden’s slippage in Iowa, saying the former vice president’s team has been less visible since the Polk County Steak Fry in September.

“We don’t see as much of a staff presence or organizing presence here like we’re seeing from Elizabeth Warren, Pete Buttigieg and Bernie Sanders. Even the Steve Bullock campaign has had more activity in the last couple of weeks than Joe Biden,” Bagniewski said. “What that tells me is he is pulling back a little bit in Iowa and concentrating on New Hampshire and South Carolina. So it’s not surprising to me his numbers are dropping.”

The Biden campaign, however, pointed to the fact that it has three campaign offices in Polk County, two of which opened after the Steak Fry, and that it has one of the most robust paid staffs in the state — more than 100 staffers — which will continue to grow until caucus day.

Campaign aides say they also just launched a $4 million digital and TV ad buy in Iowa. On top of that, the Biden super PAC, Unite the Country, aims to air TV ads in the next several weeks, and digital ads could come before that.

Patrick Bonsignore, director of digital advertising for the Biden campaign, said the campaign has consistently run ads on digital platforms including Hulu, ESPN, Pandora, Spotify, YouTube and Facebook. The ads in early states, he said, delve into Biden’s biography and, the campaign is using viewership data to find ways of targeting new Biden voters and sustain their interest.

“The best resource of our campaign is the vice president’s biography, the deep empathy for people and the way he’s fought for them and his vision for the future,” Bonsignore said. Digital spending, the campaign said, will ramp up week to week until the Feb. 3 caucuses.

The theory of the case

Biden advisers argue no candidate has taken more flak in the media than the former vice president and yet he’s still stabilized his national numbers as scrutiny shifts to Warren and others in the field.

In early September, Warren started breaking into the lead in some national polls. She built enthusiasm and drew huge crowds at the same time Biden struggled in debates and with fundraising. But Biden has since rebounded and even gained ground in a raft of recent national surveys.

Some analysts argue that national numbers can quickly shift based on the outcome in Iowa and New Hampshire. In 2008, poll after poll showed Barack Obama trailing Hillary Clinton in South Carolina. That changed once Obama proved his electability in Iowa.

“Obviously they are looking at it as a long-distance race,” Peter Hart of Hart Research Associates said of Biden's campaign. “But everything is altered after the first two contests.”

Still, recent New York Times analyses show Democrats want a moderate nominee. And in a head-to-head analysis, surveys shows Biden beating Trump in nearly every battleground state, while Warren would lose to or, is even with, Trump in those places.

“I think one of the things that isn't covered in Democratic primaries, is the fact that this is actually a race for delegates,” Biden campaign manager Greg Schultz said on a recent donor call. “Looking at a national poll and general election, you want to see where are the battleground numbers. … By the time Super Tuesday is done, then we're going to have a really good cross section of the Democratic Party in various regions.”

Schultz again managed expectations on Iowa. “I think because our coalition is so broad we're the one campaign that does not have to win Iowa,” he said. “We feel we can win Iowa, we're playing to win. Joe Biden is spending a lot of time there. But our faith is in that broad coalition.”