Sen. Bernie Sanders greets supporters in a Labor Day parade on Sept. 7 in Milford, N.H. (Charles Ommanney/The Washington Post)

Bernie Sanders is fast expanding his political staff, crafting a delegate strategy and cultivating a vast volunteer corps and digital fundraising network that he believes can seriously challenge Hillary Rodham Clinton for the Democratic presidential nomination.

Dismissed only a couple months ago as a fringe candidate, the self-described democratic socialist senator from Vermont has proven in recent weeks that he is a contender to win the Iowa caucuses and New Hampshire primary. Now Sanders is plotting his path to the nomination in what he anticipates will be a long race for delegates.

With Clinton falling in the polls and top Democrats increasingly concerned about her electability, Sanders is trying to take advantage by assembling a grass-roots machine modeled in part after Barack Obama’s 2008 campaign. Yet many serious obstacles stand between him and the nomination — and Sanders’s advisers acknowledge that their calculations would be complicated further if Vice President Biden enters the race.

The growing Sanders operation in the early states now nearly rivals the Clinton campaign. He has 54 paid staffers in Iowa and 38 in New Hampshire and dozens more coming on elsewhere, compared with 78 field staffers in Iowa and 50 in New Hampshire for Clinton. Both are far larger than any Republican campaign.

Sanders is also moving swiftly to expand his presence in South Carolina and Nevada and boost his standing among black and Latino voters. He is organizing in four states — Colorado, Massachusetts, Minnesota and Vermont — with primaries or caucuses on March 1 that his team considers opportunities for victory. He also is targeting Illinois, Michigan and Ohio, which vote later in March and where his advisers think his appeal to working-class whites can be decisive.

The Sanders campaign is training tens of thousands of volunteers to organize in their communities, expanding its social media presence and erecting an online fundraising apparatus to fully exploit every spurt of excitement. The senator is preparing to make major policy announcements — as Clinton has done this summer — designed to go deeper than the insurgent agenda he lays out in his stump speech.

Sanders also is beginning to press the case to Democratic leaders that he, not Clinton, would be the strongest nominee because of the enthusiasm his populist message generates. His advisers have been encouraged by recent polls showing Sanders trouncing Clinton among voters under 30 years old or whose annual family incomes are less than $30,000; both demographics traditionally vote in low numbers.

“He’s the base expander,” said Sanders campaign manager Jeff Weaver.

“You start communicating his message with people who haven’t heard it yet, it will resonate. We have a lot of crawl space here,” added Tad Devine, Sanders’s top strategist.

Weaver and Devine laid out the Sanders playbook in a nearly 90-minute, on-the-record interview with The Washington Post this week at the campaign’s office on Capitol Hill. A key part of their calculation, as for Obama in 2008, is amassing delegates in populous states like Texas even if Clinton wins. According to Democratic rules, delegates are allocated proportionately based on vote totals by congressional district.

Sanders’s swift rise — the latest public polls show him leading Clinton in New Hampshire, statistically tied with her in Iowa and gaining on her nationally — has startled the political establishment. His unabashedly progressive message of taking on “the billionaire class” has drawn thousands of people — in some cases tens of thousands — to his rallies. Clinton, by contrast, generally campaigns in smaller venues and has sometimes struggled to fill them.

“He has already surpassed expectations,” said David Axelrod, a former top adviser to President Obama. “Bernie has been very effective. He’s completely authentic, he is earnest, and he’s been talking about these issues all his life. But the challenge now is to transform excitement and energy into delegates and palpable progress.”

Sen. Bernie Sanders (I-Vt.), a White House contender in 2016, is known for his stances on budget issues and war. Here are his takes on Obamacare, Social Security and more. (Julie Percha/The Washington Post)
Serious hurdles

Weaver and Devine were candid about the hurdles for Sanders. One of the biggest is his low standing with minority voters, especially African Americans, relative to Clinton. She and her husband, former president Bill Clinton, have spent decades cultivating relationships with blacks and other key Democratic constituencies.

“Bernie’s not well known in the African American community — period. That’s an issue,” Weaver said.

Conceding the difficulty in persuading black political leaders to switch their allegiances from Clinton to Sanders, Devine said Sanders is targeting celebrities, pastors and community figures for support.

Sanders, 74, will try to share his personal story with black voters this fall, including on a four-day southern swing starting Saturday: He grew up poor in a Brooklyn tenement, fought for civil rights as a college student and has advocated loudly for economic and social justice through three decades in elected office. He also hopes his calls for a $15-an-hour national minimum wage and free college tuition will resonate.

Asked if there was a strategy to “humanize” Sanders like the Clinton campaign has, Devine burst out laughing. “Is he going to change to earth tones?” Devine said. “Is he going to take the pens out of his pocket? No. This is it. Nothing’s changing.”

Dan Pfeiffer, a former senior adviser to Obama, said Sanders has performed “phenomenally” but identified a vulnerability with his core message.

“The existential challenge is his campaign to date has been an inherent critique of the Democratic Party leadership, including President Obama, and the voters he has to win over are those most loyal to President Obama,” Pfeiffer said. “How he squares that circle will determine his fate.”

Many Democratic leaders have doubts about Sanders’s viability. Of the roughly 700 superdelegates, who have a say in the nominating contest, more than 400 are supporting Clinton, her campaign says. None has publicly pledged to Sanders so far. Superdelegates are free to change their allegiances, however.

The Sanders candidacy is built to be fueled by momentum. Without it, he could wither; Weaver and Devine acknowledged that he probably must win Iowa or New Hampshire or both.

By contrast, Clinton has a stronger foundation to withstand early tumult. Her campaign sees a series of Southern states with March contests as a firewall — an idea the Sanders team scoffed at.

“If you have a firewall, that means you need a firewall,” Devine said.

Weaver chimed in: “We’re going to show that prairie fire beats firewall.”

Yet another obstacle is more personal: Sanders has not faced the kind of media scrutiny, let alone attacks from opponents, that leading candidates eventually experience. Sure to follow his summer surge is an autumn of investigations that could reveal new details about his personal background and record.

“These campaigns run in phases, from the phase where people are expressing themselves passionately about issues to where they’re drilling down and asking tough questions about, ‘Okay, is this person equipped to be president of the United States?’ ” Axelrod said. “The tests become more exacting.”

A ‘bottom-up structure’

This summer, the enthusiasm Sanders stirred overwhelmed his minimalist campaign infrastructure. To catch up, officials are building what Weaver called a “cutting-edge distributive organizing program” for the more than 140,000 people who have signed up to volunteer. National field staffers are being hired to direct volunteer leaders in a state-by-state tiered network, who in turn will manage other volunteers in their regions.

Roughly 47,000 volunteers have hosted a house party, canvassed neighborhoods, written computer code for the campaign or other activities.

At Sanders rallies, volunteers armed with clipboards work the perimeters, gathering contact information from attendees. The campaign hits them up for contributions. Weaver was circumspect about detailing the campaign’s fundraising — Sanders says he has more than 400,000 contributors, with an average of about $31 — but the campaign expects to post an eye-popping total after the quarter ends Sept. 30.

“The Clinton people are trying to max people out,” Devine, noting the legal maximum donation of $2,700 per person. “Our approach to the fundraising is the exact opposite. The money needs to come bottom-up, not top-down. . . . The bottom-up structure, if it’s exploited properly, can provide a whole new wave of resources.”

Unlike Clinton, Sanders has not begun airing costly television advertisements. On Aug. 4, when Clinton went on the air in Iowa and New Hampshire, Sanders huddled with Weaver and Devine at the Capitol Hill office to weigh whether to match her on TV.

They decided not to. Instead, they husbanded most of their resources and made what Devine called an aggressive digital ad buy in the same markets — Manchester, Des Moines, Cedar Rapids and others. The effort targeted voters with a short ad that clicked through to a long biographical video about Sanders.

“People are not watching 5-and-a-half seconds of it; they’re watching 5-and-a-half minutes of it, from beginning to end — the whole thing,” Devine said. “If I pay a few cents for that while our opponent is paying several million dollars [for TV ads], we are gaining a significant tactical advantage.”