How Bernie Sanders Is Attracting Monster Crowds (and Whether Hillary Should Be Worried)

— -- "Bernie-mentum." No one is quite sure how to spell it, but Bernie Sanders supporters definitely feel the growing momentum for their underdog candidate.

“Hopefully, Webster dictionary will recognize it as a new word soon,” said volunteer Tyson Manker of Jacksonville, Illinois, who drove five hours to help at Sanders’s massive rally in Madison, Wisconsin, on Wednesday night.

Sanders drew the biggest crowd of any of the 2016 presidential contenders to date at Madison’s Alliant Energy Center: a whopping 10,000 people, according to the Associated Press.

Campaign spokesperson Michael Briggs called the turn-out impressive but not a surprise. “We had been getting indications all along that there was that much interest,” Briggs told ABC News over the phone Thursday morning.

The event was not an anomaly either. In June, 5,500 people came out to see Sen. Sanders in Denver, Colorado. In May, another 3,500 people attended a rally in Minneapolis, Minnesota, for Sanders. And approximately 5,000 people gathered in April in his hometown of Burlington, Vermont, for his campaign launch, roughly the same number who attended frontrunner Hillary Clinton’s campaign kickoff event in New York City.

“Also impressive,” Briggs added, “In Rochester, Minnesota, this morning -- on a Thursday morning -- we had 600 people for an hour-long town hall meeting,” The list of these smaller, but still relatively impressively well-attended events goes on and on. In the end of May, 300 people turned up for an event for Sanders in Kensett, Iowa, a rural town where only around 240 people live.

The campaign gauges interest in upcoming events based on RSVPs through their website and has had to change venues on more than one occasion based on a large number of people signed up to attend. It has already changed its venue for an event in Portland, Maine, on Monday, where the campaign expects more than 5,000 people to attend.

All this buzz is translating to movement in the polls, too. According to a Quinnipiac poll out today, the independent Vermont senator now trails Clinton (52–33 percent) among likely Democratic Iowa caucus goers. And in New Hampshire, WMUR has Sanders within eight points of Clinton (43-35), when just two months ago a previous poll there had him down by over 20 points.

Sanders does not have a PAC and he says he does not want donations from corporations. Still, according to a note out from the campaign today, he has raised an impressive $15 million since launching his campaign on April 30. They say that total comes from 250,000 individual donors, with the average donation size around $33 dollars.

“Some campaigns have a machine, Sanders has a ground swell of support where people are doing their part,” Mankner said.

“It is fundamentally different kind of campaign,” he continued. “People are taking ownership. ... There is no centralized leader.”

Sanders likes to tell the crowds that come out to see him that his campaign is not about him, but is instead “a political revolution.” It is a message that resonates with grassroots organizers from labor groups, the Occupy Wall Street movement, and the former "Ready for Warren" campaign that has now pivoted to Sanders. These organizers use their online networking experience to amplify his message and effectively get the word out for upcoming events.

“A lot of people have been disenfranchised from the system and Bernie is a candidate that people can rally behind,” said Shana East, an artist and activist from Chicago who helped organize the website and social media campaign "People for Bernie," which now has sub-chapters for almost every state and major city.

“Bernie is a candidate that people can rally behind. He has a lot of integrity. They feel like he is not just another political puppet,” she added. “People are coming out of the woodwork who want to get involved. We are helping to empower them so they feel like their role is important.”

Through the group’s website, those interested in Sanders can organize a "meetup" event and "People for Bernie" will help promote it. They are also taking the time to train people on how to use Twitter and Facebook. The group successfully facilitated 99 events (symbolic of “the 99 percent”) in the first week of Sanders's campaign and have held nationwide conference calls every two weeks since.

Super volunteers like East see themselves as playing a key role traditionally reserved for a paid campaign staffer. “We knew how important it was for the media to see how many people are backing Bernie,” she said about the event in Madison. “We have artists making memes, making videos and sharing it.”

“We can show visually this isn’t some crackpot candidate. He has a following. It is a movement," she said.

Although Sanders would be the oldest president ever elected, he has an impressive social media presence himself. His Senate Facebook page has over 1.3 million likes and his campaign page is catching up, with over 700,000. In addition, his campaign has adopted a method of signing up supporters at events for text message alerts.

Sanders’s Iowa director, Pete D'Allesandro, said the mega-events and his on-the-ground effort in the Hawkeye State go hand-in-hand. “They help build enthusiasm for us here,” he said. “Because of social media, you can see the increase in the Bernie and Iowa supporter pages.”

Last Sunday, Sanders told ABC News' George Stephanopoulos he was riding the momentum all the way to the White House.

“We are going to win New Hampshire. We're going to win Iowa and I think we're going to win the Democratic nomination, and I think we're going to win the presidency,” he said.