There are 1,681 voting precincts in the state of Iowa, and Hillary Clinton's campaign had identified at least one supporter in each of them by August.

The feat was so remarkable it prompted Democratic strategist Brad Anderson – the architect of President Barack Obama's 2012 re-election effort in the state – to call up Clinton's headquarters and ask how her team pulled it off.

"You have to understand, some of these precincts only have four or five Democrats in them, but they found a supporter," Anderson says. "They had an organizer waiting in one person's driveway to sign them up when they got home from work."

Seared by her experience in 2008 when she finished a discomfiting third in Iowa's Democratic presidential caucuses, the 2016 Clinton campaign early on built a state-of-the-art, door-to-door ground-game organization designed to mirror the Obama juggernaut that foiled her eight years ago.

Yet now, months of well-laid planning, relentless canvassing and dry-run practice caucusing is being jeopardized by a surge toward Bernie Sanders, the self-described democratic socialist who is betting his progressive message of revolutionary change can overcome even the best machinery.

"In a way, it's remarkable, Hillary's done everything I thought she should do. But he's not far behind," says Tom Miller, the state's attorney general and a Clinton supporter. "He could win. This is close."

Clinton sounded a new sense of urgency during a speech in Indianola, Iowa, on Thursday, assailing Sanders as a vessel for unrealistic ideas. "Theory isn't enough," she said. "A president has to deliver in reality."

But reality could bite for Clinton come caucus day on Feb. 1. And Jerry Crawford – a prominent Des Moines, Iowa attorney, Democratic player and Clinton booster who stays in contact with the candidate – is under no illusions about the increasingly competitive contest.

"My hat's off to Bernie Sanders. If you had told me a self-avowed socialist could make the race in Iowa competitive, I would have been pretty dubious of that," he says.

The latest Des Moines Register/Bloomberg Politics poll, which has an impeccable record tracking the caucuses and was released last week, found Sanders trailing Clinton by just 2 points. It also showed his personal favorability 3 points higher than hers and revealed that the gap between the rich and poor – the cause Sanders has built his campaign around – stood at the top of the issue list for Democratic caucusgoers, along with the economy and civil rights.

Sanders' team has been racing to make up its organizational deficit to Clinton in the closing weeks, ramping up phone-banking, door-knocking and the busing in of volunteers. But the central question surrounding the Vermont senator's chances of pulling off an Iowa upset is whether he's Barack Obama or Howard Dean.

In 2008, Obama helped lure a record number of roughly 240,000 Iowans to the caucuses, blowing by historical expectations for turnout. In 2004, despite raucous rallies and bands of hardcore supporters, Dean's coalition never turned up and he flopped into third place.

The Sanders campaign has proudly touted the size of its crowds, noting they had brought out more than 34,000 people to Iowa events through December, with the goal of drawing close to 50,000 by Feb. 1.

Grant Woodard, who worked on Clinton's 2008 Iowa effort, says no one on that campaign believed Obama's rally-goers would ultimately show up and put in the time to caucus.

He believes Clinton still holds an advantage, but thinks Sanders' message of sweeping institutional change – not dissimilar to Obama's – could be compelling when neighbors are standing arm-to-arm inside a school auditorium conversing about their caucus decisions.

"Clinton's message has been a little bit too narrow in some people's minds," Woodard says. "He's talking about a wholesale change of the political structure – he calls it a revolution, calls the whole system corrupt. With her, it's been much more calculating, sort of the standard Clinton, 'Let's talk about narrow issues and policy proposals.' It's harder to excite people over what the Clinton campaign's selling."

This may explain why the Clinton team is increasingly turning to the electability argument against Sanders.

Clinton allies note that while their candidate has understandably been weakened by sustained Republican attacks and incessant press scrutiny for more than a year, Sanders has incurred little incoming fire.

"They haven't thrown a single punch at him. When they do, it won't be pretty," Crawford says. "I think it's a very huge question mark as to what happens when they come at him full-bore. We know she can take it; with him it's an unknown."

Asked whether he thought Sanders could win a general election if he became the nominee, Crawford replies coyly: "I think there are two things you have to be to be a great president. One of them is president."

But Woodard agrees with the premise that electability can sway those who show up to caucus and are still persuadable. "With people still undecided, the big hurdle you have to clear is who can actually win the presidency," he says.

Polling has shown Sanders' support to be clustered in urban areas around colleges, whereas Clinton's is more evenly divided throughout the state. But given the rules of delegate allocation by precinct, a wider reach is better than domination in a populous city or rural county.

"Whether 300 people show up or 400 people show up, you still get the same delegates," Anderson notes.

This is where Clinton's organizational muscle could prove crucial to holding off Sanders' momentum.

Bret Nilles, Democratic Party chairman for Linn County – the second-most populous county in the state – says he fields calls from anyone who needs to schedule a ride on caucus day. Of the 15 he's fielded over the last couple days, he says "100 percent have been elderly individuals who are all Hillary supporters."

"The Clinton group has built up more relationships," he says. "These 18- to 30-year-olds are with Bernie. But with a caucus, you have to commit to being out there two or three hours. We want you to come, but you've got to stay. I'm not sure that's going to resonate with everybody."

The public nature of the caucuses – walking into a neighborhood church, school or gymnasium at 7 p.m. and publicly expressing support for one candidate over the other – is the X-factor that not even the most seasoned political strategist can forecast.

Caucus veterans talk about the unsuspected resonance of a neighbor's opinion and how it can grant permission to change one's mind inside the room.

"This is 1,681 neighborhood meetings. It matters that I know who's in the room. It matters that they've seen us. It matters that we've talked at the door," says a Clinton campaign caucus consultant not authorized to speak on the record. "The science of it is, we know what we're doing. The art of it is, we don't know what they're doing."

What the Clinton team does know is that Sanders is on the march, and given his strength in New Hampshire, a loss to the senator in Iowa would stir some level of panic.

"It's a bit odd for the cycle that it even got this close," Woodard says. "You've got a guy who is a socialist going toe-to-toe with not only one of the most recognizable people in the country, but the world. It just goes to show how fed up people are with any establishment politician."

But Miller, who was on his way to Clinton's Indianola, Iowa, event on Thursday when he spoke to U.S. News, says he was carrying a calming message he hoped to deliver personally to the former secretary of state.

"If for some reason she loses both [Iowa and New Hampshire], I still think she's going to be the nominee. These are unusual states without minority populations," he says. "So I think if she hangs in there, she'll be the nominee.