Presidential candidate Sen. Bernie Sanders speaks during the live webcast Wednesday in Washington. (Katherine Frey/The Washington Post)

Presidential hopeful Bernie Sanders spoke to supporters at more than 3,500 parties around the country Wednesday night, trying to harness the enthusiasm of the large crowds he has been drawing and bolster his insurgent campaign organization.

Standing in a packed living room in a modest Southwest Washington apartment, Sanders told his supporters over a live stream on YouTube that “all of us are part of making history.

The senator from Vermont asked for help with door knocking, phone banking and some of the other basics of campaigning, and offered a stinging critique of the country’s problems as he sees them.

“No, it is not acceptable that the rich gets richer and everyone else gets poorer,” said Sanders, a self-described democratic socialist. “Enough is enough.”

Nationwide, more than 100,000 people signed up to attend one of the Sanders parties. His message was heard at a sports bar in Chicago, a yoga studio in Houston, a union hall in Harrisburg, Pa., an artists colony in Los Angeles and hundreds of homes across the nation, including dozens in the Washington area.

The outreach comes at a key juncture for Sanders, who faces a better funded and organized Democratic primary rival in front-runner Hillary Rodham Clinton.

Although Sanders has drawn bigger audiences than any of the hopefuls from either party, those who cheer as he rails against the political power of the “billionaire class” include many young people and progressives who have not participated in politics before.

The 73-year-old firebrand faces the same test that has undercut other candidates who have surged early in presidential races: keeping supporters engaged.

“How do you keep these people with you?” said Dante Scala, a political science professor at the University of New Hampshire. “Can you build an infrastructure to hold the weight of your supporters?”

Nowhere will the challenge be more acute than in Iowa, the nation’s first caucus state. Unlike in New Hampshire and other primary states, caucus voters need to show up at a prescribed time and stay for a few hours to be heard.

“It takes a lot of care and feeding to get your supporters to show up,” said Dennis Goldford, a political scientist at Drake University in Des Moines. “If someone is left to their own devices for a few months, they might not even go. You really need to keep the flame lit.”

For campaigns, that means having the staff in place to continually check in with likely caucus-goers, by phone and in person, to make sure they are still on board.

The Sanders campaign has been scrambling to hire more staff and provide more volunteer opportunities, after initially being overwhelmed by the level of support pouring in. “We’re still catching up to the demand,” campaign manager Jeff Weaver said. “We’re not quite there.”

He described Wednesday night’s gatherings — held in 3,520 locations, according to a tally on the event Web site — as “playing to our strength.”

At one party, in famously left-leaning Takoma Park, Md., about 20 people nodded and clapped as Sanders bemoaned the influence of money in politics and talked about expanding social programs.

An hour after the live stream, attendees were still talking and had agreed on a course of action: another meet-up next month with tentative plans to create a bumper sticker for the Takoma Park area and to recruit more volunteers.

“The most impactful thing you can do is get those three people outside of this room and get them to sign up for this campaign,” said Greg Beyna, 35.

Claire Sanberg, a Sanders aide who spoke after the senator, told supporters that the campaign needs help with “unglamorous but essential work.”

During the quarter that ended last month, Clinton outraised Sanders by $47 million to $15.1 million. The fundraising imbalance is reflected in the size of the candidates’ respective campaign staffs.

Clinton’s recently filed campaign finance reports showed 340 staff members and 60 field organizers on the payroll. Weaver said the Sanders team is paying about 60 people total, half of them field organizers.

But Sanders is steering a lot of his money to building operations in early nominating states. He has opened 11 offices throughout Iowa — the same number as Clinton.

Clinton, meanwhile, has six offices open in New Hampshire to Sanders’s one. But Sanders plans to open a half-dozen more shortly, aides say.

Hosting hundreds of house parties on a single night is not a new strategy for campaigns, although Sanders’s aides point out that they are doing it two months earlier in the cycle than those who came before them.

In September 2003, former Vermont governor Howard Dean called in to 3,466 parties across the country in his quest for the Democratic nomination. The “House Call” set a Guinness world record at the time for the largest conference call.

Supporters of then-candidate Barack Obama hosted more than 4,000 house parties on a single day in June 2008.

Veteran Democratic strategist Joe Trippi, who ran Dean’s 2004 campaign, said the turnout Wednesday night for Sanders underscored the passion that progressives feel for him.

“These are pretty energized people, and the value he gets is foot soldiers to go door to door,” Trippi said. But one of the larger challenges Sanders faces­, he said, is reaching out to other segments of the Democratic Party.

In the 2008 race against Clinton, Obama started out as the insurgent progressive candidate but was able to win the nomination by successfully wooing establishment Democrats — something Sanders hasn’t done.

Obama also benefited from out-organizing Clinton in Iowa, which “she never saw coming,” Goldford said.

Elizabeth Koh contributed to this report.