DES MOINES — Andrew Yang bounded on the stage here like he always does, grinning and grooving to “Return of the Mack,” that slinky, old-school ‘90s jam that has served as his signature walk-on music since the early summer.

The song was an expertly unconventional choice for 2020’s most unconventional candidate. And just as that old R&B single slowly attained pop immortality as a feel-good mainstay on any decent party playlist, Yang, too, has become a kind of sleeper contender in the race for the Democratic presidential nomination.

Written off early on by party insiders who refused to take seriously a political unknown like him, the entrepreneur and former nonprofit executive has assembled an intense grass-roots following that has helped him attract some of the biggest crowds in the race, raise more than $16.5 million in the last quarter and outlast better-known candidates in the field, including three senators and two governors.

On Monday night, there was more evidence of the Yang phenomenon. About 700 people, many of them dressed in the campaign’s signature MATH hats and “Yang Gang” T-shirts, were crammed into a second-floor ballroom here at Drake University, a massive crowd on a cold, snowy evening for a candidate who barely gets a fraction of the news coverage of his Democratic rivals.

“This is enormous! This is incredible!” Yang called out as he took the stage, sounding, as he often does, as shocked as anybody at how far his candidacy has gotten.

But there was something bittersweet about it all. The event took place just a few buildings away from the campus theater where presidential hopefuls will gather Tuesday night for the seventh debate of the primary season, the last before Iowa’s first-in-the-nation Feb. 3 caucuses. But Yang won’t be there, having failed to meet the Democratic National Committee’s rules of entry.

To qualify for the debate, candidates needed to hit 5 percent in four polls approved by the DNC either nationally or in the early-voting states (Iowa, New Hampshire, Nevada and South Carolina) or 7 percent in two early state polls plus raise money from 225,000 individuals including 1,000 donors in at least 20 states.

Yang easily met the fundraising goal but failed to meet the polling threshold, having hit 5 percent in just two polls, including Friday’s Des Moines Register/CNN/Mediacom Iowa poll. He and his team repeatedly complained about the rules, pointing to the lack of polls conducted during the qualifying period.

On Monday, Yang told reporters he shared his supporters’ “frustration” that he wouldn’t be on the debate stage and criticized the DNC’s lack of flexibility. “I want us to be on that stage,” he said. “I think we earned it.”

But Yang and his supporters are making the best of it.

His campaign organized a debate-eve rally that attracted a crowd far larger than those who have turned out recently for other candidates. On Tuesday, Yang’s supporters are organizing Yang National Visibility Day, sending people to knock on doors and wave signs across the country — including Iowa, where he and his team are hoping to surprise people with a better-than-expected showing in the caucuses.

Not unlike Sen. Bernie Sanders (I-Vt.), Yang is banking on a surge of new voters, including young people and Iowans across party lines who may not have participated in a caucus before. Unlike other campaigns, his has appeared to invest very little in the kind of traditional field operation that finds and gets those voters out to caucus. Yang’s campaign organizers are hoping that will happen organically, just as his supporters across the state have worked on their own for months to get the word out about his candidacy.

Using his surge of contributions, most of which came from small-dollar donors, Yang has invested about $3 million to air television ads across Iowa in recent weeks trying to increase his name identification among voters. One ad touts his signature proposal of handing out $1,000 a month to all Americans older than 18 as a way of stimulating and remaking the economy, including in rural America. Another ad tackles the cost of prescription drugs. But it’s unclear whether the ads are helping his candidacy.

On Monday night, very few people in the crowd raised their hands when Yang asked whether they had come to the event after seeing one of his ads. “Oh,” Yang replied, a hint of amusement in his voice. “We spent of a lot of money on those ads. This is actually quite upsetting.”

But there are tangible signs that Yang has sparked something. In recent months, other candidates have adopted some of Yang’s talking points. That includes Biden, who has raised the threat of automation on job loss and cited some of the same examples Yang has, such as the development of self-driving trucks on the trucking industry.

Advisers for rival campaigns say they have detected positive signs for Yang in their private polling, including praise from voters who like that he doesn’t sound like a politician and is talking about issues that other candidates are not.

In recent months, Yang has sought to increase his support and be taken more seriously in the nomination race — expanding his campaign operation to include more traditional staffers, like a new media team, and bringing on advertising consultants who previously worked for Sanders. The concern is that Yang’s absence from the debate stage will hurt whatever progress he has made — although many supporters who showed up Monday night argued it would have no impact at all.

“He got so little speaking time at the last debates that I think it won’t matter at all,” said Mary Parsons, a 72-year-old retiree from Des Moines who had been planning to caucus for Buttigieg before switching to Yang in the fall.

Parsons, who caucused for Sanders four years ago, said she was drawn by Yang’s positivity and the fact that his message wasn’t partisan and could draw support beyond Democrats. “What I like about him is that he’s person who doesn’t say, ‘I’ve got to get this group. I’ve got to get that group.’ He just speaks his truth and is trying to talk to and help and win everybody,” she said. “That’s how you beat Donald Trump.”

Onstage, Yang was his usual self, bouncing between arguing serious policy and offering the self-depreciating jokes he’s become known for on the campaign trail. Celebrating his 45th birthday, he beamed as supporters serenaded him with “Happy Birthday” and later presented him with a piñata in the shape of a $100 bill with his face on it and a cake that read “POTUS 46.”

At one point, he praised Iowans for their diligence in vetting the Democratic field and the role they play in democracy.

“Most of our fellow Americans look up and see our government as a series of pipes that are just clogged full of money, clogged with millions and millions of dollars of lobbyist cash,” he said. “And they despair around our country that there is nothing they can do to change it. Americans are smart. They can generally do nothing to change it. It is going to be up to you all to change it on February Third and flush the pipes clean.”

He predicted that his campaign “would shock the world.” “We’re going to make history on February Third, and they will never see us coming,” he said.