After the Democratic candidates in Florida’s special elections burned through millions and millions of dollars on the way to double-digit losses this week, some Democrats are asking where that money deluge came from — and where it all went.

The answer to both questions is, in part, a 23-year-old law student and dungeon master — in Dungeons & Dragons — with a lucrative side gig.

In between classes and fantasy play, Jackson McMillan is also the chief executive of Key Lime Strategies, a small fund-raising firm in Florida that scored big when it landed as clients the two Democratic nominees in the Florida congressional elections, Josh Weil and Gay Valimont. Mr. McMillan said they had combined to raise $25 million.

“We’ve built a juggernaut,” he said in an interview.

Along the way, Mr. McMillan has piled up critics far beyond his years. Much of the focus is on his unusual fee structure, which one top party official excoriated in a cease-and-desist letter as “exorbitant.” His firm received a 25 percent cut of “true profits” — the proceeds after fund-raising expenses — for both special elections.

Mr. McMillan is unapologetic.

“A lot of the people who are critiquing me online are mad that it wasn’t them,” he said of raising so much money, which he said put a scare into Republicans and injected real money into long-neglected corners of a rightward-drifting state.

One secret ingredient to his firm’s success, Mr. McMillan explained, is Dungeons & Dragons.

“All the senior fund-raising strategists at my firm — myself, Ryan — we’re dungeon masters,” he said of his college friend and the firm’s chief operating officer, Ryan Eliason. “We run Dungeons & Dragons games. So we weave narratives and tales. It’s like our biggest hobby. We basically tell a really compelling story. And that’s what sets us apart from — that and a lot of technical analysis — is what sets us apart from some of our competitors.”

Others say the story his team spun up about Mr. Weil and Ms. Valimont made him a false-hope merchant who cashed in on the desperation of small Democratic donors wanting to fight the new Trump administration. These were lopsidedly Republican seats, which the G.O.P. won by more than 30 percentage points last fall and where Democrats faced near-impossible odds; the Republicans won by 14 percentage points on Tuesday.

Stefan Smith, a digital strategist who is head of digital engagement at the American Civil Liberties Union, called the 25-percent-of-profits fee structure “absurd” and said the races had diverted donor money from more urgent priorities under false pretenses of competitiveness.

“Democrats are experiencing the largest trust gap we’ve experienced in a generation, and we are not going to win that back by letting predators roam freely across the digital ecosystem,” Mr. Smith said, speaking in his personal capacity. “It is on all of us to hunt them to extinction.”

There is no single standard for fund-raising contracts, but more typically, consultants earn a retainer and either a percentage of what is spent creating and placing ads, or a much smaller percentage of what is raised overall.

So just how much did Mr. McMillan’s firm clear?

“I don’t think I’m totally comfortable sharing that,” he said, waving off talk that it had amounted to a multimillion-dollar payout and saying that all of the bills had yet to be settled.

“Don’t get me wrong,” he added. “My firm did well.”

Records show that by mid-March, the two campaigns had paid his firm $4.7 million, roughly 38 percent of their total spending.

Much of the money sent to Key Lime Strategies appears to have paid for fund-raising ads.

In the first 90 days of the year, Mr. Weil’s campaign was the single biggest political spender on Instagram and Facebook in the nation, spending $2.5 million. Ms. Valimont’s campaign was close behind, at $2.1 million.

Neither Mr. Weil nor Ms. Valimont returned calls for comment. Both sent written statements praising Mr. McMillan. Mr. Weil said the campaign’s payments to the company had covered polling and mailers, as well as email, text and social media messaging.

“The work he did on this campaign should cement Jackson McMillan as the gold standard for Dem fund-raising and political coordination in the state of Florida for years to come,” he said. Ms. Valimont said the funds helped to boost “voter registration efforts that would never have garnered any investment under normal circumstances.”

It’s an adage of online political fund-raising that you have to spend money to make money. (And raising big money brings more media attention, which in turn can bring in more money.) The question is if quite that much needed to be spent. Records show the advertising blitz overwhelmingly went to raising more money rather than persuading Florida voters.

Both Mr. Weil and Ms. Valimont, for instance, spent far more on ads in California than in Florida, records show.

All told, the Weil campaign spent far less on local television ads, $1.5 million, than out-of-state online fund-raising.

At one point in the race, Representative Alexandria Ocasio-Cortez, Democrat of New York, said she was being featured in fund-raising appeals without her permission. And lawyers for David Hogg, a Democratic National Committee vice chair, wrote a cease-and-desist letter asking Mr. McMillan to pull ads featuring Mr. Hogg because he would not “lend his name to fund-raising efforts that divert substantial portions of the proceeds from a campaign to cover exorbitant fees for fund-raising consultants.”

Mr. Hogg went even further in a post on X. “People like Jackson McMillan are the exact type of consultants who people say are the problem in our party,” he wrote.

In an interview, Mr. Hogg explained his decision to go after Mr. McMillan by name: “Nothing is going to change until we start calling these people out.”

Mr. McMillan said that the episode had been a “misunderstanding” and that the firm had pulled the ads and apologized. He noted that he and Mr. Hogg, 24, had risen in Florida politics at the same time and are of the same generation.

“We’re in the same space,” Mr. McMillan said. “And I would love to work together with Vice Chair Hogg more, and I think we have the same motives and goals, which is why I was very, very surprised to see his onslaught of attacks.”

Mr. McMillan is also the treasurer of the Florida Future Leaders PAC, a youth-organizing group formed last year. State records show the PAC paid Key Lime Strategies more than $534,000, roughly 65 percent of the group’s total expenses.

Mr. McMillan defended his firm’s pay structure, which is listed on its website, as cheaper and “more ethical” than some rivals, who sometimes take a smaller cut of the total raised, regardless of what the campaign is netting.

Mr. McMillan said he had actually stumbled into the digital fund-raising business.

He was once an aspiring paleontologist at the University of Florida, where he said he had enrolled early as a 15-year-old after skipping some grades. But a trip to Wyoming for a dinosaur-bone dig was interrupted by a car accident, and he recalled rethinking his career choice as he removed glass shards from his arm.

He met his business partner and current roommate, Mr. Eliason, in college. They formed the Magic the Gatoring club, where students gathered to play the fantasy card game Magic the Gathering, and a quick bond followed.

Mr. McMillan filed the paperwork for Key Lime Strategies in June 2022 and began doing political field programs for local races, including some for the Tampa City Council. “It was a lot of work for not a lot of payoff,” Mr. McMillan recalled of early fund-raising efforts.

But then came Ms. Valimont’s first long-shot bid for Congress, in 2024 against Matt Gaetz — a high-profile villain for many Democrats. Mr. McMillan, by then a full-time student, said it had been the “perfect contest” to experiment in.

Ms. Valimont raised $1.58 million. More than half — $812,824.15 — went to Key Lime Strategies.

She lost by 32 percentage points.

Then she ran in the special election, rehired Key Lime Strategies, raised millions more and lost again.

If fund-raising doesn’t work out, Mr. McMillian is already testing another business that he filed the paperwork for in January: using artificial intelligence to spot consumer complaints for potential lawsuits against “corporate bad actors.” “That is the kind of law that I am most familiar with,” he said, citing some courses and an internship last summer.

Either way, he is betting on himself — and his Gen Z colleagues.

“I will put money on a 20-something in politics every day over someone who’s been doing this for 40 years,” Mr. McMillan said. “Give them an energy drink, and they will outwork you 10 to one.”

Kitty Bennett contributed research.