Hillary Clinton meets with Iowa voters at B.R. Miller Middle School in Marshalltown on Tuesday. (Melina Mara/The Washington Post)

Despite her immediate hurdles in the Iowa caucuses and New Hampshire primary, Hillary Clinton has been quietly expanding her political networks in states that come later on the presidential calendar, confident that she can deny insurgent Bernie Sanders the Democratic nomination by swamping him in a state-by-state delegate slog.

Many in Clinton’s Iowa staff of more than 100 are expected to quickly fan out to the two dozen states holding primaries and caucuses in March. Already, her campaign has held more than 10,000 volunteer training sessions, phone banks, debate watch parties and other organizing meetings to recruit and mobilize supporters in those places.

Outside the Clinton campaign, there is mounting anxiety among senior Democrats and some Clinton allies that the “March firewall” she had been counting on to stop Sanders would be vulnerable if he were to score early wins.

With Clinton’s recent focus squarely on the first four states — Iowa, New Hampshire, Nevada and South Carolina — her campaign’s activities to organize in the March states have gone largely unnoticed. Her strategists have been reluctant to reveal too much of their playbook publicly so as not to tip off her opponents.

But senior Clinton campaign officials this week detailed the breadth of the operation in the March states and beyond, eager to show that their efforts are sophisticated and that her campaign is durable enough to withstand any early loss to Sanders, the Vermont senator who is surfing a wave of populist frustration to contend for the nomination.

“The early states are obviously very important in terms of kick-starting the primary, but they [award] only 4 percent of the delegates,” campaign manager Robby Mook said. “Over half the delegates will be chosen by the end of March. So we’ve always been focused on making sure that we can build the delegate lead that we need over the course of March.”

Many Democrats, including Clinton’s advisers, are mindful of how quickly the landscape changed for her in 2008 after Barack Obama’s Iowa victory. Though Sanders does not yet have the national coalition Obama enjoyed, wins by him in Iowa and New Hampshire would at least complicate Clinton’s calculus for securing the nomination.

Some Clinton backers are concerned that momentum could carry him to a better-than-expected finish in South Carolina, where Clinton’s deep support among black voters gives her an advantage, and across the South and Midwest in March.

“If Sanders wins Iowa and New Hampshire, the energy and the media coverage are going to be substantial — and all of a sudden that idea that organizational strength outweighs energy starts to disappear,” said one well- ­connected Democrat familiar with the Clinton campaign’s organization, who requested anonymity to share private worries.

Clinton’s recent polling decline in Iowa has led some allies to worry whether she could lose her dominance in the South — even in Arkansas, where she once served as first lady. In 2008, she swept Arkansas with 70 percent of the primary vote over Obama. But this time, the electorate is angrier and less predictable, and her allies have spotted a smattering of Sanders yard signs in Little Rock.

“There were a lot more Arkansas voters in 2008 that were acquainted with Hillary Clinton than there are in 2016,” said Skip Rutherford, a longtime Clinton loyalist and dean of the University of Arkansas Clinton School of Public Service, who has known the Clintons for decades. “The population here has changed. I think she will carry the state on the Democratic side, but I would be surprised if it was by the same margin.”

The Fix's Chris Cillizza previews the Iowa caucuses, looking at what the outcome could mean for both Democrats and Republicans. (Peter Stevenson/The Washington Post)

Marlon Marshall, Clinton’s director of state campaigns and political engagement, said the campaign is taking nothing for granted.

“I don’t believe in firewalls,” Marshall said. “We have strong support in certain demographics, but that doesn’t mean it’s a firewall. We have to go out and earn support. We can’t just say, ‘Oh, I think we’re good here.’ ”

Some top Democrats are confident in the Clinton operation — and openly skeptical about Sanders’s ability to translate the energy of his large crowds into votes at caucuses and primaries.

“Enthusiasm is not to be underestimated, but enthusiasm that isn’t channeled into actual voting is really just a bunch of rallies,” said David Axelrod, the top strategist on Obama’s 2008 and 2012 campaigns. “That’s not enough to win a nomination.”

Though Sanders began organizing far later than Clinton, he recently deployed paid staff to all 11 of the “Super Tuesday” March 1 states to organize supporters on the ground. And he can rely on his small-dollar fundraising machine to power him through a protracted nomination battle.

Meanwhile, Sanders allies, such as Progressive Democrats of America, have been hosting liberal activists at house parties and phone banks to lay the groundwork for him in such states as Alabama, Georgia and Virginia.

Clinton also has outside help, including many major labor unions and other interest groups, such as Planned Parenthood, who are backing up their endorsements with manpower in key states.

“We are on fire to make her the next president of the United States,” said Mary Kay Henry, president of the Service Employees International Union. “We have an unprecedented [number of] volunteers going into Iowa and New Hampshire and Nevada, Minnesota, Colorado and Virginia. And then we’re getting ready for our volunteer program in Pennsylvania, Ohio, Florida, Michigan and Wisconsin.”

Clinton’s long-game strategy is a purposeful departure from 2008, when she focused mainly on organizing in the early states. As the nomination dragged on following her Iowa loss, Clinton scrambled to organize, especially in caucus states, and her campaign lurched from one contest to the next.

Axelrod said this was a “fatal miscalculation” and that she has learned her lesson: “They are fortified for a longer fight if they need to be in one.”

Jeremy Bird, who helped lead Obama’s grass-roots organizing, said he sees parallels eight years later in Clinton’s campaign.

“It’s putting staffers on the ground and really being maniacally focused on the caucus math and the calendar,” Bird said. “I don’t think they’ll get caught off guard on the delegate math like they did in 2008 — and that’s a big difference.”

Clinton’s advisers have long insisted they thought the primary would become competitive. At the moment Sanders announced his candidacy last spring, Mook was holding a meeting in his office at the campaign’s Brooklyn headquarters and told aides that he viewed Sanders as a genuine threat, according to someone in attendance.

Last June, Mook convened senior staff for a strategy meeting to map out plans to organize in the March states. By Labor Day, he had hired staff in many of those states.

In Colorado, for instance, which holds a March 1 caucus and, like Iowa, awards delegates based on complicated rules, paid Clinton operatives have been on the ground for five months, signing up and training precinct leaders.

They recruited nearly 70 politicians and other activists to join Clinton’s state leadership council. In October, they held a 300- ­person organizing meeting led by Gov. John Hickenlooper. And on Jan. 9, some 250 uber-volunteers from across the state came to Denver for a Clinton precinct captain convention.

“To be successful in a caucus, you need to build an organization, and you can’t do that overnight,” said Brad Komar, Clinton’s Colorado state director. “You can’t arrive two weeks beforehand and expect to have success.”

The Clinton campaign has made similar pushes elsewhere. On Friday, it ran a phone bank in Yarmouth, Maine, where the caucuses are March 6. This month, volunteers held a phone bank in Marietta, Ga., home to a March 1 primary. And this week, “organizing academies” were held in Austin, Dallas, Houston, San Antonio and the Rio Grande Valley to train volunteers to canvass ahead of the March 1 Texas primary.

For Mook and his lieutenants, a key is not just running strong in big primary states, many of which Clinton won in 2008, but in the smaller caucus states where she got out-hustled by Obama.

“We’ve made a real priority of organizing in those caucus states, deep into the calendar,” Mook said. “We’re not going to get blown out this time. We have a plan to win.”

Matea Gold and Tom Hamburger in Washington contributed to this report.